DOC: vars: document dump_all_vars() sample fetch

DOC: vars: document dump_all_vars() sample fetch

Add documentation for the dump_all_vars() sample fetch function in the
configuration manual. This function was introduced in the previous commit
to dump all variables in a given scope with optional prefix filtering.

The documentation includes:
- Function signature and return type
- Description of output format
- Explanation of scope and prefix arguments
- Usage examples for common scenarios

This completes the implementation of GitHub issue #1623.