doc fix for dynamic::operator[]
doc fix for dynamic::operator[]

Summary:
folly::dynamic's operator[] was documented as never invalidating
iterators.  In the object case when a key is implicitly inserted the
underlying unordered_map can rehash, which does invalidate the iterators.
This diff amends the comment to reflect this case.

Reviewed By: yfeldblum

Differential Revision: D7085861

fbshipit-source-id: 41ce0497bc60dba9f4529d10010f3b92f9a6c2b3

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