Update changelog and issue tracker notes for the enhanced
`default_completion` behavior.

When a concrete entry's custom completion handler produces a `Suggest`
value, combine it with the default subcommand suggestions instead of
replacing them entirely. `FileCompletion` still falls back to the
default completion since it cannot be meaningfully merged.

Fix command tree matching when global flags precede the subcommand.

Add `#[completion(EntryFallback)]` syntax to enable custom
completion suggestions for unmatched entries, merging fallback
results with default completions via `Suggest::combine()`.

Rename the internal fallback type and its associated `ProgramCollect`
plumbing from `ErrorDispatcherNotFound`/`build_dispatcher_not_found` to
`EntryFallback`/`build_entry_fallback`, along with the generated enum
variant and pack type. Update all documentation, examples, and tests
accordingly.