[Rd] Possible Update to R-internals Manual

[brodie gaslam]

According the R-ints the only current uses of the `truelength` meta datum is for environment hash tables.  Jim Hester just made me aware that R3.4.0 introduces a new use case: growable vectors.

I attach a patch to the R-ints manual that reflects this change.  The wording is obviously just a suggestion.  Additionally, it may be worth moving the footnote into the main body of the document now that there are more use cases.

Best,

Brodie.
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