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/20220616174840.1202070-8-shy828301@gmail.com


Signed-off-by: Yang Shi <shy828301@gmail.com>
Reviewed-by: Zach O'Keefe <zokeefe@google.com>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Miaohe Lin <linmiaohe@huawei.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>