devguide should mention rules about "paragraph reflow" in the documentation

[brettcannon]

Originally from http://bugs.python.org/issue23320

[willingc]

Next action: add a brief section in docquality.rst perhaps under the section "Helping with Documentation issues"

• small documentation changes for content is preferred
• reflowing text is discouraged as it makes review more difficult (a core dev will reflow text if needed or ask you to reflow in a follow up PR)

[willingc]

Closing as #1018 will cover this issue.