I am attaching a patch to fix this thing. The change is very small, but 
when documenting it, I discovered some inconsistencies in the manual 
regarding shell completion options. I did my best to fix them. Now the 
documentation changes are bigger than the functional change.

Below I pointed several things that may need attention:

- Whether such broad changes in manual may be done together with a lisp 
change (new variable)
- Whether an unnumbered subsubsection Shell Mode Completion Options in 
Shell Mode Options is a good idea
- Whether anchors can be used, because our info viewer does not follow 
them correctly
- Whether this new variable may be introduced despite the feature freeze
- Whether a new position in concept index (shell completion) is fine

Suggested commit message:

Introduce a variable shell-completion-cur-dir and document it.
Document a related detail in exec-path variable.

Add some consistency in the documentation of completion in the manual:
- add a link from Completion section to Shell Mode,
- move documentation of shell-completion-fignore from Shell Mode to Shell
   Mode Options,
- group Shell Mode Completion Options into a new unnumbered subsubsection.

FIXME: anchors used in this commit currently do not work correctly in Emacs
info viewer, however work correctly in Linux info viewer, so this must be
a bug in our info viewer.

     * lisp/shell.el (shell-completion-cur-dir): New variable to allow shell
     completion to use filenames from current directory.
     (shell--command-completion-data): Use it.
     * src/callproc.c (Vexec_path): Documentation of the library path in it.
     * doc/mini.texi (Completion Options): Add a link to Shell Mode 
Completion
     Options.
     * doc/misc.texi (Shell Mode): Document new variable
     shell-completion-cur-dir.
     Move documentation of shell-completion-fignore from Shell Mode to Shell
     Mode Options.
     Add an unnumbered subsubsection to group completion options. This also
     required to move pushd paragraph above this group.