R1:67f015a76c79

Clarify STRICT_ALL_TABLES instructions

Summary:
- Mention the `[mysqld]` section.
- Should be `sql_mode`, not `sql-mode`.
- Improve clarity of the text.
- Slightly nicer markup.

Test Plan: {F147076}

Reviewers: csilvers, btrahan

Reviewed By: btrahan

Subscribers: epriestley, spicyj

Differential Revision: https://secure.phabricator.com/D8847