bash-completion: Fix spurious 'following' target completion candidate

The '--target' completion parsed 'cmake --build --target help'
with a sed pattern whose leading '...' matched any three
characters, so the header line yielded a bogus 'following'
candidate.  Anchor the pattern to literal dots.

Also note in the user-interaction guide that 'make' target
tab-completion is provided by the external bash-completion
package, which may hide the plain '<target>' name when a
'<target>/fast' variant exists, and recommend
'cmake --build --target' completion instead.

Fixes: #27873

Auxiliary/bash-completion/cmake

@@ -131,7 +131,7 @@ _cmake()
                 COMPREPLY=( $( compgen -W "$target_options" -- "$quoted" ) )
             else
                 local targets=$( cmake --build "$build_dir" --target help 2>/dev/null \
-                    | sed -n -e 's/^... \([^ ]*\).*$/\1/p' \
+                    | sed -n -e 's/^\.\.\. \([^ ]*\).*$/\1/p' \
                        -e '/^\[/d' -e 's/^\([^ :]*\):.*$/\1/p' | \
                 grep -v '^/' | sort -u )
                 COMPREPLY=( $( compgen -W "$targets $target_options" -- "$quoted" ) )

Help/guide/user-interaction/index.rst

@@ -618,6 +618,18 @@ are not rebuilt if out of date.  The :ref:`Ninja Generators`
 are sufficiently fast at dependency checking that
 such targets are not provided for that generator.
 
+.. note::
+
+  Shell tab-completion of ``make`` target names is provided
+  by the external ``bash-completion`` package, not by CMake.
+  Some versions of it hide the plain ``<target>`` name when a
+  ``<target>/fast`` variant also exists, treating the ``/`` as
+  a directory separator.  The targets themselves remain
+  available to build.  To complete project target names
+  reliably, use :option:`--target <cmake--build --target>`
+  completion instead, e.g. tab-completion after
+  ``cmake --build <dir> --target``.
+
 :ref:`Makefile Generators` also provide build-targets to
 preprocess, assemble and compile individual files in a
 particular directory.