This adds string-x.rst for each subcommand x of string. The main page
(string.rst) is not changed, except that examples are shown directly after
each subcommand. The subcommand sections in string.rst are created by
textual inclusion of parts of the string-x.rst files.
Subcommand man pages can be viewed with either of:
```
man string collect
man string-collect
string collect <press F1 or Alt-h>
string collect -h
```
While `string -h ...` still prints the full help.
Closes#5968
Instead of requiring a flag to enable newline trimming, invert it so the
flag (now `--no-trim-newlines`) disables newline trimming. This way our
default behavior matches that of sh's `"$(cmd)"`.
Also change newline trimming to trim all newlines instead of just one,
again to match sh's behavior.
The `string collect` subcommand behaves quite similarly in practice to
`string split0 -m 0` in that it doesn't split its output, but it also
takes an optional `--trim-newline` flag to trim a single trailing
newline off of the output.
See issue #159.
I hope this is now complete.
Also, shorten enough descriptions to make `string match --<TAB>`
show a two column pager with 80 cols.
We really should have shown more retraint in the design of `string`,
not all of the flags required both a long and short option created.
