Fixed incomplete docs on ASCII formatting.

[sun]

Problem

• The SHELL_PIPE env variable does not work for the db command.

Proposed solution

1. Clarify docs for how to retain ASCII table formatting with the db command.
2. Clarify docs for how to use the env variable, because not all users understand how env variables are specified; the only correct example I found was in the original issue https://github.com/wp-cli/wp-cli/issues/1102.

[codepuncher]

I have found that SHELL_PIPE is also ignored when using a command such as wp @production db size --human-readable --all-tables