Update `suggest_edits` prompt to clarify usage of `<old_text>` when
using update/create operations using update/create operations.

- Add a mention that `old_text` is required for all but create.
- Change definition of `create` operation to also mean overwrite, as
some models heavily prefer rewrites.
- Remove mention of `If this tag is not specified, then the entire file
will be used as the range.` which is not current behavior.


Closes #22340

Release Notes:

- N/A (not sure if this requires a release note)