Merge handbook documentation to API: skip_on_empty process plugin

Problem/Motivation

Process plugin documentation is being moved from the Handbook to the API documentation of the process plugin class.

The handbook page for process plugin contains more information than the API documentation of the plugin.

Before we delete the handbook page and redirect it to the API page, we need to either enhance the API doc or decide that it is OK to discard the additional information currently found in the handbook.

The difference is this:
--clips--
The handbook page gives an example use case for both plugin 'methods'.

For 'row' method:

• API doc: "Skips the entire row when an empty value is encountered."
• Handbook in addition to this: "This is useful when migrating parent values. When the parent source value is 0 or empty, which represents the root element, there is no need to set a destination value. In the other case a parent exists and needs a migration lookup."

For 'process' method:
API doc: "Prevents further processing of the input property when the value is empty."
Handbook in addition to this: "This is useful when combined with a migration process plugin to check if a related item was previously migrated."

(The handbook page also has more examples than the API page but the additional examples do not add much value as they only repeat what has been demonstrated already.)

When writing a patch for this, a couple of other issues were identified that should be fixed a the same time:

• The example was using the 'migration' process plugin. This process plugin has been renamed to 'migration_lookup'.
• API documentation standards: No newline between example code and the descriptive paragraph.
• Needs minor improvements to phrasing.

Proposed resolution

Copy these pieces of 'this is useful' instructions to the API documentation.

Remaining tasks

Do it.

User interface changes

N/A.

API changes

N/A.

Data model changes

N/A.