Merge handbook documentation to API: static_map 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--

YAML Booleans
Using an unquoted string which represents a boolean value in YAML (various forms of yes/no/true/false/on/off) as a key in your map will use that boolean value rather than the string itself as your key, which is almost certainly not what you want. Be sure to quote any such strings to use them as strings.

Doesn't work:

process:
  bar:
    plugin: static_map
    source: foo
    map:
      # This works as if it were '1: to'.
      TRUE: to

Does work:

process:
  bar:
    plugin: static_map
    source: foo
    map:
      'TRUE': to

Periods in key names
Mapping from a string which contains a period does not work at all:

process:
  bar:
    plugin: static_map
    source: foo
    map:
      'Version 3.0': version_3_0

This will give error:
exception 'Drupal\Core\Config\ConfigValueException' with message 'Version 3.0 key contains a dot which is not supported.' in /var/www/example/docroot/core/lib/Drupal/Core/Config/ConfigBase.php:211

There is an existing issue #2827897: static_map process plugin does not work with periods in keys to address this. In the meantime, a custom process plugin would be needed to map source data containing periods if they could not be removed by a previous process plugin like machine_name (which will run the value over the transliteration service, lowercases it, replaces anything that's not a number or a letter with an underscore and removes duplicate underscores):

process:
  bar:
    -
      plugin: machine_name
      source: foo
    -
      plugin: static_map
      map:
        'version_3_0': version_3_0

When writing the patch to include the additional documentation listed above, it was identified that there were several other areas needing improvements:

• API documentation standards: how default values are indicated.
• API documentation standards: use single quotes instead of double quotes.
• It is very confusing which descriptive text belongs to which example.
• There are many places where the phrasing can be clarified.

Proposed resolution

• Include the 'YAML booleans' and 'periods in key names' topics in the API documentation
• Fix API documentation standard violations.
• Harmonize the descriptions so that the descriptive text is always above the code example.
• Clarify the phrasing where possible

Remaining tasks

Do it.

User interface changes

N/A.

API changes

N/A.

Data model changes

N/A.