RT4351: Update doc for OPENSSL_cleanse

OPENSSL_malloc_init,

OPENSSL_malloc, OPENSSL_zalloc, OPENSSL_realloc, OPENSSL_free,

OPENSSL_clear_realloc, OPENSSL_clear_free,

OPENSSL_clear_realloc, OPENSSL_clear_free, OPENSSL_cleanse

CRYPTO_malloc, CRYPTO_zalloc, CRYPTO_realloc, CRYPTO_free,

OPENSSL_strdup, OPENSSL_strndup,

OPENSSL_memdup, OPENSSL_strlcpy, OPENSSL_strlcat,

OPENSSL_clear_realloc() and OPENSSL_clear_free() should be used

when the buffer at B<addr> holds sensitive information.

The old buffer is filled with arbitrary data by calling OPENSSL_cleanse()

The old buffer is filled with zero's by calling OPENSSL_cleanse()

before ultimately calling OPENSSL_free().

OPENSSL_cleanse() fills B<ptr> of size B<len> with a string of 0's.

Use OPENSSL_cleanse() with care if the memory is a mapping of a file.

If the storage controller uses write compression, then its possible 

that sensitive tail bytes will survive zeroization because the block of 

zeros will be compressed. If the storage controller uses wear leveling, 

then the old sensitive data will not be overwritten; rather, a block of 

0's will be written at a new physical location.

OPENSSL_strdup(), OPENSSL_strndup() and OPENSSL_memdup() are like the

equivalent C functions, except that memory is allocated by calling the

OPENSSL_malloc() and should be releaed by calling OPENSSL_free().