Explain the issues of setting fish as login shell

Also stop explaining this in three places. In particular this removes
an FAQ entry.

Fixes #8078

doc_src/faq.rst

@@ -343,24 +343,6 @@ This also means that a few things are unsupportable:
 - Non-monospace fonts - there is *no way* for fish to figure out what width a specific character has as it has no influence on the terminal's font rendering.
 - Different widths for multiple ambiguous width characters - there is no way for fish to know which width you assign to each character.
 
-How do I make fish my default shell?
-------------------------------------
-If you installed fish manually (e.g. by compiling it, not by using a package manager), you first need to add fish to the list of shells by executing the following command (assuming you installed fish in /usr/local)::
-
-    echo /usr/local/bin/fish | sudo tee -a /etc/shells
-
-
-If you installed a prepackaged version of fish, the package manager should have already done this for you.
-
-In order to change your default shell, type::
-
-    chsh -s /usr/local/bin/fish
-
-
-You may need to adjust the above path to e.g. ``/usr/bin/fish``. Use the command ``which fish`` if you are unsure of where fish is installed.
-
-Unfortunately, there is no way to make the changes take effect at once. You will need to log out and back in again.
-
 .. _faq-uninstalling:
 
 Uninstalling fish

doc_src/index.rst

@@ -56,15 +56,30 @@ Once fish has been installed, open a terminal. If fish is not the default shell:
 
     > exit
 
+.. _default-shell:
+
 Default Shell
 -------------
 
-To make fish your default shell:
+There are multiple ways to switch to fish (or any other shell) as your default.
 
-- Add the line ``/usr/local/bin/fish`` to ``/etc/shells``.
-- Change your default shell with ``chsh -s /usr/local/bin/fish``.
+The simpler is to set your terminal to start fish. See its configuration and set the program to start to ``/usr/local/bin/fish`` (if that's where fish is installed - substitute another location as appropriate).
 
-For detailed instructions see :ref:`Switching to fish <switching-to-fish>`.
+The more involved and complete way is to set fish as your login shell. To do that, you need to:
+
+1. Add the shell to ``/etc/shells`` with::
+
+    > echo /usr/local/bin/fish | sudo tee -a /etc/shells
+
+2. Change your default shell with::
+
+    > chsh -s /usr/local/bin/fish
+
+Again, substitute the path to fish for /usr/local/bin/fish - see ``command -s fish`` inside fish. To change it back to another shell, just substitute ``/usr/local/bin/fish`` with ``/bin/bash``, ``/bin/tcsh`` or ``/bin/zsh`` as appropriate in the steps above.
+
+.. warning::
+
+   Setting fish as your login shell may have issues, because some operating systems (including a bunch of linux distributions) only do some of their configuration in shell startup files like /etc/profile. So you could notice e.g. $PATH being wrong, and you would have to redo that setup.
 
 Uninstalling
 ------------

doc_src/python_docs_theme/static/pydoctheme.css

@@ -111,7 +111,7 @@ div.documentwrapper {
     float: none;
 }
 
-div.document, div.body {
+div.document, div.body, div.warning {
     background-color: inherit;
     color: inherit;
 }

doc_src/tutorial.rst

@@ -29,7 +29,7 @@ which means you are all set up and can start using fish::
 
 This prompt that you see above is the fish default prompt: it shows your username, hostname, and working directory.
 - to change this prompt see :ref:`how to change your prompt <prompt>`
-- to switch to fish permanently see :ref:`switch your default shell to fish <switching-to-fish>`.
+- to switch to fish permanently see :ref:`Default Shell <default-shell>`.
 
 From now on, we'll pretend your prompt is just a ``>`` to save space.
 
@@ -765,29 +765,6 @@ Now in another shell::
     vim
 
 
-.. _switching-to-fish:
-
-Switching to fish?
-------------------
-
-If you wish to use fish (or any other shell) as your default shell,
-you need to enter your new shell's executable in two places.
-
-Add the shell to ``/etc/shells`` with::
-
-    > echo /usr/local/bin/fish | sudo tee -a /etc/shells
-
-
-Change your default shell with::
-
-    > chsh -s /usr/local/bin/fish
-
-This assumes you installed fish to /usr/local/bin, which is the default location when you've compiled it yourself. If you installed it with a package manager, the usual location is /usr/bin/fish, but package managers typically already add it to /etc/shells. Just substitute the correct location.
-
-(To change it back to another shell, just substitute ``/usr/local/bin/fish``
-with ``/bin/bash``, ``/bin/tcsh`` or ``/bin/zsh`` as appropriate in the steps above.)
-
-
 Ready for more?
 ---------------