Fixes for gdb stubs (#13169)

* gdb: Clarify a comment
* gdb: Fix gdb.unwinder.Unwinder.__call__ argument. It takes a gdb.PendingFrame, not a gdb.Frame.
* gdb: Unwinders may implement a proto without subclassing gdb.unwinder.Unwinder
* gdb: Fix Breakpoint.__init__

 1. `line` should be `int|str`, not just `int` (IDK what a string means,
    but that it can be a string is clear if you read
    py-breakpoint.c:bppy_init().
 2. `type` argument should be able to be passed to the "location" form,
    not just the "spec" form, even if
    https://sourceware.org/gdb/current/onlinedocs/gdb.html/Breakpoints-In-Python.html
    neglects to mention it (don't worry, I'll be submitting a patch to fix
    the doc soon).
 3. Fix the positional argument order (based on GDB's sources, it isn't
    really documented)
 4. Use more `@overloads` to enforce that at least 1 of `function`,
    `label`, or `line` are given in the location form.

stubs/gdb/METADATA.toml

@@ -1,5 +1,5 @@
 version = "15.0.*"
-# This is the official web portal for GDB,
+# This is the official web portal for the GDB Git repo,
 # see https://sourceware.org/gdb/current/ for other ways of obtaining the source code.
 upstream_repository = "https://sourceware.org/git/gitweb.cgi?p=binutils-gdb.git;a=tree"
 extra_description = """\