[paulbalandan]

Description
Sweeps user_guide_src/source for language errors, RST problems that would block the Markdown migration, and line width.

• Fixes grammar and awkward wording, and corrects statements that did not match the code.
• Wraps unmarked code in double backticks and unmarked paths in double asterisks, and fixes broken RST constructs.
• Wraps prose at 120 characters, in a separate commit that changes only line breaks. Changelogs, tables, literal blocks and field lists are not reflowed.
• About 25 headings are retitled, so their anchors change.

make html has 0 warnings, the rendered pages are identical before and after the reflow, and the docs-update converter passes on all 325 pages.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide