docs: Fix remaining "\subsection" markup

See #5696.

[ci skip]

sphinx_doc_src/cmds/fish_vcs_prompt.rst

@@ -16,7 +16,8 @@ If a vcs isn't installed, the respective function does nothing.
 
 For more information, see their documentation.
 
-\subsection fish_vcs_prompt-example Example
+Example
+-------
 
 A simple prompt that displays vcs info::
 

sphinx_doc_src/cmds/source.rst

@@ -31,6 +31,7 @@ Example
     # Causes fish to re-read its initialization file.
 
 
-\subsection Caveats
+Caveats
+-------
 
 In fish versions prior to 2.3.0 the ``$argv`` variable would have a single element (the name of the sourced file) if no arguments are present. Otherwise it would contain arguments without the name of the sourced file. That behavior was very confusing and unlike other shells such as bash and zsh.

sphinx_doc_src/cmds/string.rst

@@ -59,7 +59,8 @@ The following subcommands are available.
 
 ``string join`` joins its STRING arguments into a single string separated by SEP, which can be an empty string. Exit status: 0 if at least one join was performed, or 1 otherwise.
 
-\subsection string-join0 "join0" subcommand
+"join0" subcommand
+------------------
 
 ``string join`` joins its STRING arguments into a single string separated by the zero byte (NUL), and adds a trailing NUL. This is most useful in conjunction with tools that accept NUL-delimited input, such as ``sort -z``. Exit status: 0 if at least one join was performed, or 1 otherwise.
 
@@ -113,7 +114,8 @@ Exit status: 0 if at least one replacement was performed, or 1 otherwise.
 
 See also ``read --delimiter``.
 
-\subsection string-split0 "split0" subcommand
+"split0" subcommand
+-------------------
 
 ``string split0`` splits each STRING on the zero byte (NUL). Options are the same as ``string split`` except that no separator is given.
 
@@ -344,7 +346,8 @@ Match Regex Examples
     <outp>0xBadC0de</outp>
 
 
-\subsection string-example-split0 NUL Delimited Examples
+NUL Delimited Examples
+----------------------