Fix #2574, document software bus timeout clock behavior

[sylvesterkaczmarek]

Fix #2574

Document the platform-specific behavior of finite CFE_SB_ReceiveBuffer and CFE_SB_ReceiveBufferWithRoute timeouts when the underlying POSIX queue uses an absolute real-time clock.

The Software Bus known-problem note now explains that wall-clock adjustments while a finite receive is pending can cause an early timeout or a longer wait, and advises applications that require elapsed-time semantics to account for this platform limitation.

Testing:

• documentation-only change
• verified the branch contains only the intended update to docs/src/cfe_sb.dox

Expected behavior changes:

• none; this documents existing platform behavior

Contributor: sylvesterkaczmarek