MFC r317288,r317289:

MFC r317288,r317289:

r317288:

libgeom(3): apply minor polish

- Use .Dv when mentioning NULL per mdoc(7).
- Reword `g_device_path`, `g_open_by_ident`, and `g_providername`'s descriptions
  so they're less wordy.
- Fix a typo in `g_device_path` (can not -> cannot).

Tested with:	igor, make manlint

r317289:

libgeom(3): note that stdio.h is required when referencing gctl_dump(3)

gctl_dump(3) is only exposed when stdio.h is #include'd first, per its
addition in r112510. The reasoning noted for the conditional "exposure"
of the function was to "limit #include pollution".

This addresses an issue I found with the documentation when looking at
bug 218809, which in turn addresses a -Wimplicit-function-declaration
compiler warning in `tools/regression/geom_gpt/test.c` (it uses
gctl_dump(3)).

git-svn-id: svn://svn.freebsd.org/base/stable/10@319003 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f