Please document required alignment for modp_b64_decode()

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What steps will reproduce the problem?
1. Call modp_b64_decode() with src or dest not 4-byte aligned.
2. Code hangs when the unaligned pointer is cast to uint32 (srcInt/destInt).

What is the expected output? What do you see instead?
On inspecting the code, I understand this is expected. I'm just asking for it 
to be documented in the function comments.

What version of the product are you using? On what operating system?
ARMCC compiler in Keil uVision. MQX 4.1.0 OS.

Please provide any additional information below.

Original issue reported on code.google.com by cp.en...@gmail.com on 1 Apr 2015 at 9:58