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).

MFC after:	5 weeks
Tested with:	igor, make manlint
Sponsored by:	Dell EMC Isilon

lib/libgeom/libgeom.3

@@ -28,7 +28,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd December 4, 2010
+.Dd April 22, 2017
 .Dt LIBGEOM 3
 .Os
 .Sh NAME
@@ -324,8 +324,8 @@ The
 .Fn g_device_path
 function returns the full path to a provider given a partial or full path to the
 device node.
-If the device can not be found or is not a valid geom provider, NULL is
-returned.
+.Dv NULL
+is returned if the device cannot be found or is not a valid geom provider.
 .Pp
 The
 .Fn g_get_ident
@@ -344,20 +344,19 @@ string.
 .Pp
 The
 .Fn g_open_by_ident
-function opens provider using its ident, unlike
+function opens provider using its identification, unlike
 .Fn g_open
-which uses provider's name.
-If the
+which uses the provider's name.
+The function will store the provider's name in the
 .Fa name
-argument is not
-.Dv NULL ,
-the function will store provider's name there.
+parameter if it is not
+.Dv NULL .
 .Pp
 The
 .Fn g_providername
 function returns the provider name of an open file descriptor.
-If the file descriptor does not point to a valid geom provider, NULL is
-returned.
+.Dv NULL
+is returned the file descriptor does not point to a valid geom provider.
 .Pp
 All functions except
 .Fn g_providername