Document thread-safety aspects of UMF API and performance implications

[vinser52]

Rationale

Most UMF APIs are thread-safe and designed for multi-threaded environments. Our documentation should explicitly state thread safety for each UMF API function where applicable. Also, we should describe the performance implications of calling an API concurrently from multiple threads.

Description

Most of our API is thread-safe (where applicable) the API description part of our documentation for each API function should explicitly state whether it is thread-safe or not.

Also for thread-safe function, we should describe the performance implications of calling the function concurrently. Ideally, performance-critical functions that are supposed to be called on a critical path should not impose global locking/waiting.