gdb/doc: the all-doc build target should build .... all docs

I noticed that the 'all-doc' build target doesn't build all the doc
formats, 'man' and 'html' are missing.

This commit updates 'all-doc' so that all formats are built.

This doesn't change the default 'all' target, which is the default
target used when building GDB itself, the 'all' target continues to
just build the 'info' docs.

There should be no difference in the actual generated output after
this commit, I'm just changing what gets built.

Approved-By: Tom Tromey <tom@tromey.com>