help: don't crash in keyword search if an extension fails to provide docs
Not all external extensions provide docs; if you use such an extension, you
will experience a crash if you use "hg help --keyword <word>", and <word>
happens to match the extension name.
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
for software developers.
Basic install:
$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing:
$ make local # build for inplace usage
$ ./hg --version # should show the latest version
See https://mercurial-scm.org/ for detailed installation
instructions, platform-specific notes, and Mercurial user information.