Update: Add JSDoc documentation to content model hierarchy (fixes #852)

[joe-replin]

Fixes #852

Update

• courseModel.js — Added @file, @module, and @class documentation describing its role as the top-level course root node
• contentObjectModel.js — Added module and class documentation for the shared base of pages and menus
• pageModel.js — Added module and class documentation, clarifying that pages are leaf-level content objects containing articles
• menuModel.js — Added module and class documentation; added JSDoc for setCustomLocking() describing its recursive locking behaviour
• articleModel.js — Added module and class documentation for the article tier of the content hierarchy
• blockModel.js — Added module and class documentation for the block tier
• componentModel.js — Added module and class documentation, including a description of the compact attempt-state array format [numbers[], booleans[], arrays[]] used for SCORM tracking
• mpabc.js — Added module and class documentation for the MPABC bootstrap controller, including a side-effects import example and description of the Adapt.contentObjects / articles / blocks / components collections it creates

Testing

1. Open each changed file and confirm JSDoc block comments parse without syntax errors (no unclosed /** blocks)
2. Run grunt dev and verify no build errors are introduced by the documentation additions
3. Optionally generate API docs and confirm module and class entries appear for all eight files