run-in-tempdir: Copy doc/* only if needed

To preserve disk space on copying all files to a temporary directory,
the “doc” directory is only copied when building documentation. A
comment describing the need is also added to Makefile.am, as well as a
superfluous direct invocation of Bash removed.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>