civibuild.md - Add documentation about CIVIBUILD_PATH and custom-types

In this page and other `tools/*.md` pages, the sections use basic
capitalization.

Update Hooks Pre and Post documentation indicating Domain is now aval…

civilint.md - Split out comments on multiple lines

These examples are in a code block  - which prevents wrapping. When reading on the published site, you have to scroll horizontally.

Moving them to separate lines makes them more legible.

Updated docs for dev/core#1077

Adding examples to markdown documentation.

Explain difference between ts and E::ts a bit more

Updated docs for dev/core#870

transactions.md - Add more intro. Add API discussion.

Improve hook_civicrm_alterMailParams re: headers and flexmailer. fix #654

Co-Authored-By: Mikey O'Toole <mikey@mjco.uk>

Co-Authored-By: Mikey O'Toole <mikey@mjco.uk>

Co-Authored-By: Mikey O'Toole <mikey@mjco.uk>

theme.md - Add more examples of URL/CSS manipulation

Resolve #549 Document Long and short cache

Fix spelning misteaks

Add mention of how to schedule an api

Co-Authored-By: Mikey O'Toole <mikey@mjco.uk>

Explain confidential checkbox on gitlab issues