sanitizers: Add documentation for the no_sanitize attribute

[rcvalle]

Add documentation for the no_sanitize attribute, being stabilized in rust-lang/rust#123617 along with AddressSanitizer and LeakSanitizer.

[ehuss]

Some drive-by questions:

• I'm not clear which options to the attribute are being stabilized. The PR says "leak" and "address", but there doesn't appear to be a "leak" option. Is address the only attribute option being stabilized? If so, that needs to be documented. And this PR is referring to the thread option, which doesn't appear to be stabilized? If so, then it shouldn't include that.
• Where can this attribute be placed? It seems to only validate its usage when there is an argument, which isn't how I would expect it to work.
• What is the behavior if the attribute is specified multiple times? Does it take the union of all options?
• I assume the attribute is ignored if the target does not support the sanitizer?
• Does this only work with build-std or custom std builds?