Instead of redundantly documenting "directory" configuration option in
each source plugin, refer to the built-in functionality documentation of
the Source class.

  * source.py: Add anchor for built-in functionality.
  * sources/plugins/*.py: Remove directory config option, and add reference to
    Source's built-in functionality.

Fixes #713.

For each element plugin derived from BuildElement, add reference to the
built-in functionality documentation of BuildElement.

  * buildelement.py: Add anchor for built-in functionality.
  * plugins/elements/*.py: Add refence to BuildElement's built-in
    functionality for each build element.

Related to #713.

plugins: Don't redundantly document common configurations

Closes #713

See merge request !896

We were unconditionally inserting into the yaml cache anything we had
retrieved from the yaml cache.  This had the side-effect of being expensive
on re-runs of commands where yaml hadn't changed.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

_yaml.py: Do not insert into cache if retrieved from cache

See merge request !902