Request: Add a note about CORS allowed headers to Request docs.

Also fixes #2494.

Thanks @5HT2A for reporting the docs problem and helping to debug and verify above mentioned issue.
Browser: We only need the Browser.Plugins object in the compat section.
Browser: Fix two deprecation notices in docs.
Browser: Move the Browser.Platform object into a compat section.
Browser: Make `parse` function work without lowercased UA string.

Also removed the `ua` and `platform` local vars, and made the 1.2-compat
`Engine` object just find the userAgent on the `navigator` object.
Browser: Move `Browser[Browser.name]` into a compat block.
Browser: docs, s/backwards compatibility/backward compatibility/
Browser: Document Browser.parse.
Browser: Move and slightly rewrite docs for Browser.name, Browser.version and Browser.platform.
Browser: Fix two links in docs.
Browser: Rename Browser.parse to Browser.parseUA.
Browser: Manual Semicolon Insertion.
Browser: Grammar and consistency in docs.
Browser: Have Browser.parseUA return 'windows' rather than 'win' for Windows.