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/[e-mail]
Signed-off-by: SoumishDas <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

mm/memory.c

···
 	p4d_free_tlb(tlb, p4d, start);
 }
 
-/*
- * This function frees user-level page tables of a process.
+/**
+ * free_pgd_range - Unmap and free page tables in the range
+ * @tlb: the mmu_gather containing pending TLB flush info
+ * @addr: virtual address start
+ * @end: virtual address end
+ * @floor: lowest address boundary
+ * @ceiling: highest address boundary
+ *
+ * This function tears down all user-level page tables in the
+ * specified virtual address range [@addr..@end). It is part of
+ * the memory unmap flow.
  */
 void free_pgd_range(struct mmu_gather *tlb,
 			unsigned long addr, unsigned long end,