Improve gcore manpage and clarify "-o" option

Improve gcore manpage and clarify "-o" option

Ref.: https://bugs.debian.org/904628

It has been reported that gcore's manpage is a bit imprecise when it
comes to two things:

- It doesn't explicity say that the command accepts more than one PID
  on its CLI.

- It fails to mention that the argument passed through the "-o" option
  is actually a prefix that will be used to compose the corefile's
  filename, and not the actual filename.

I decided to give it a try and rewrite parts of the text to further
clarify these two points.  I ended up rewording the "Description"
section because, IMHO, it was a bit confuse to understand.

To make things consistent, I've also renamed the "$name" variable in
the gcore.in script, and expanded the usage text.

gdb/doc/ChangeLog:
2018-07-27  Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb.texinfo (gcore man): Rewrite "Description" and "-o"
	option sections to further clarify that gcore can take more
	than one PID, and that "-o" is used to specify a prefix, not a
	filename.

gdb/ChangeLog:
2018-07-27  Sergio Durigan Junior  <sergiodj@redhat.com>

	* gcore.in: Rename variable "name" to "prefix".  Expand
	"usage" text.