Refer to (new) `Hacl_HMAC_DRBG_free` and fix duplicate documentation.

[duesee]

This closes #296.

[coveralls]

Pull Request Test Coverage Report for Build 3637771432

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.01%) to 55.949%

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Totals
Change from base Build 3631490752:	0.01%
Covered Lines:	33744
Relevant Lines:	60312

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💛 - Coveralls

[duesee]

Oh, good catch.