Document snippet sourcing order

Fixes #3099.

doc_src/index.hdr.in

@@ -1158,6 +1158,7 @@ Configuration files are evaluated in the following order:
   - `/usr/share/fish/vendor_conf.d` (set at compile time; by default, `$__fish_datadir/conf.d`)
 
   If there are multiple files with the same name in these directories, only the first will be executed.
+  They are executed in order of their filename, sorted (like globs) in a natural order (i.e. "01" sorts before "2").
 
 - User initialization, usually in `~/.config/fish/config.fish` (controlled by the `XDG_CONFIG_HOME` environment variable).
 
@@ -1165,6 +1166,8 @@ These paths are controlled by parameters set at build, install, or run time, and
 
 This wide search may be confusing. If you are unsure where to put your own customisations, use `~/.config/fish/config.fish`.
 
+Note that ~/.config/fish/config.fish is sourced _after_ the snippets. This is so users can copy snippets and override some of their behavior.
+
 These files are all executed on the startup of every shell. If you want to run a command only on starting an interactive shell, use the exit status of the command `status --is-interactive` to determine if the shell is interactive. If you want to run a command only when using a login shell, use `status --is-login` instead. This will speed up the starting of non-interactive or non-login shells.
 
 If you are developing another program, you may wish to install configuration which is run for all users of the fish shell on a system. This is discouraged; if not carefully written, they may have side-effects or slow the startup of the shell. Additionally, users of other shells will not benefit from the Fish-specific configuration. However, if they are absolutely required, you may install them to the "vendor" configuration directory. As this path may vary from system to system, the `pkgconfig` framework should be used to discover this path with the output of `pkg-config --variable confdir fish`.