Allow inherited docstrings to be overridden

[lengau]

Example case: Rockcraft documentation.

The current (pre-kitbash) documentation says of the project's name field:

The name of the rock. This value must conform with Pebble's format for layer
files, meaning that the ``name``:

- must start with a lowercase letter [a-z];
- must contain only lowercase letters [a-z], numbers [0-9] or hyphens;
- must not end with a hyphen, and must not contain two or more consecutive
  hyphens.

This matches the craft-application field description but is more informative for rock authors. We should be able to override this in the docs without overriding the field in the source code.