docs(header): correct HeaderMap max capacity and clarify reserve panic

[ChrisJr404]

Closes #603.

The # Limitations section said a HeaderMap can hold 32,768 entries, but inserts actually start failing at 24,576 (that is usable_capacity(MAX_SIZE)), which is the number the original reporter hit. I corrected the count and pointed the section at the fallible try_insert / try_append / try_reserve methods so there is a clear way to stay under the limit without panicking.

I also reworded the reserve panic note. It referenced the private MAX_SIZE constant, so it now points at the type docs and try_reserve instead.

Docs only, no behavior change.