doc: proc: fix the description to THPeligible

The THPeligible bit shows 1 if and only if the VMA is eligible for
allocating THP and the THP is also PMD mappable. Some misaligned file
VMAs may be eligible for allocating THP but the THP can't be mapped by
PMD. Make this more explicitly to avoid ambiguity.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Yang Shi <[e-mail]>
Reviewed-by: Zach O'Keefe <[e-mail]>
Cc: Kirill A. Shutemov <[e-mail]>
Cc: Matthew Wilcox <[e-mail]>
Cc: Miaohe Lin <[e-mail]>
Cc: Vlastimil Babka <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

Documentation/filesystems/proc.rst

···
 "SwapPss" shows proportional swap share of this mapping. Unlike "Swap", this
 does not take into account swapped out page of underlying shmem objects.
 "Locked" indicates whether the mapping is locked in memory or not.
+
 "THPeligible" indicates whether the mapping is eligible for allocating THP
-pages - 1 if true, 0 otherwise. It just shows the current status.
+pages as well as the THP is PMD mappable or not - 1 if true, 0 otherwise.
+It just shows the current status.
 
 "VmFlags" field deserves a separate description. This member represents the
 kernel flags associated with the particular virtual memory area in two letter