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:

Reviewers: csilvers, btrahan

Reviewed By: btrahan

Subscribers: epriestley, spicyj

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