Add errata clarifying Exception vs. Throwable usage in PSR-3

[Aliance]

PSR-3 was written before PHP introduced the Throwable interface. At that time, Exception was the only error-related base type available.

In modern PHP versions, both Exception and Error implement Throwable, and it is common practice for logging libraries to accept any Throwable when logging failures.

This PR adds an Errata section to PSR-3 clarifying that references to Exception in the specification - specifically the exception context key - should be interpreted as allowing any Throwable instance.

The main specification text is intentionally left unchanged. This errata is meant to:

• reflect modern PHP language semantics;
• improve clarity for implementors;
• increase awareness without introducing breaking changes.

Per FIG process, this change is intended to go through the errata vote.

@Seldaek please, take a look once again if possible

Related pr: #1345

[Seldaek]

LGTM, thanks

[Aliance]

@Jean85 can you take a look, please?