Improve PluginUI documentation

Problem/Motivation

This is a followup issue for #1535868: Convert all blocks into plugins.

• #1535868: Convert all blocks into plugins adds a rough API for user interfaces that allow listing, creating, and editing plugin object instances (e.g. block instances).
• This API is almost completely undocumented, which makes it more difficult to review and use.
• An eventual goal of the blocks and layouts initiative is to replace much of the block UI with an integrated layout building UI, but if that does not happen, the current API must be documented.

Proposed resolution

• Add detailed documentation to the abstract PluginUI class explaining what it's for and how it should be used.
• Add detailed documentation to the BlockPluginUI class explaining its purpose and how it is used within the block module.
• Add inline documentation to BlockPluginUI::form()
• Add inline documentation to the block plugin UI derivative definitions in BlockPluginUI::getDefinitions().
• Add API documentation to system_plugin_ui_form(), system_plugin_autocomplete(), and system_plugin_ui_access().

Remaining tasks

Related issues

• #1875340: Review PluginUI architecture
• #1874840: Add explicit test coverage for the BlockPluginUI