appender: update crate docs to call-out lossy behavior

[f1recracker]

By default, non-blocking writers are lossy, which is an important caveat and can silently cause loss of logs which is difficult to diagnose.

Motivation

By default, non-blocking writers are lossy, which is an important caveat and can silently cause loss of logs which is difficult to diagnose.

Solution

Update crate docs to explicitly call out lossy behavior.

Closes #2865