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)).