[PATCH][BZ #12140] Add note on free list pointers overwriting M_PERTURB bytes

[Roland McGrath]

Use @theglibc{} and so forth, never "glibc".  Or just say, "the @code{free}
function".  But if there was a need to say this, then perhaps there's a
need to reword to be even more clear.  The option never guarantees that you
can find any particular byte values in freed space.  What it guarantees is
that the contents the block had before it was freed will be overwritten.