mm: add kernel-doc comment for free_pgd_range()

Provide kernel-doc for free_pgd_range() so it's easier to understand what
the function does and how it is used.

Link: https://lkml.kernel.org/r/20250325181325.5774-1-soumish.das@gmail.com


Signed-off-by: SoumishDas <soumish.das@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>