Correct the documentation for OPENSSL_MALLOC_FD

The documentation was misleading in that it suggests that this environment
variable will record information about all allocations. While this is true
it doesn't record the most useful information that you might expect such
as the requested size of the allocation! It is mainly for use in
conjunction with OPENSSL_MALLOC_FAILURES, and reports information about
what chance an allocation has of failing.

We also clarify that the mem_debug functions are actually no-ops in 3.0.

Reviewed-by: Hugo Landau <hlandau@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/18967)
