The Sphinx Emacs Lisp extension assumed all \[command] key sequence
substitutions in documentation strings are preceded by a \<mapvar>
sequence specifying the keymap, unlike substitute-command-keys which is
used when accessing docstrings within Emacs.

With this commit the actual substitute-command-keys function is used
when building docstringdb.json, instead of the Python reimplementation.
The drawback is that docstringdb.json now contains partial Sphinx
markup.

Closes #1544. Note that the generated Texinfo file cannot be removed as
it is built by Sphinx which is not available during ELPA package builds.

Better for diffs

Also regenerate