docs: improve description on Env Vars and Config values

[kenjis]

Description
Closes #7453

• improve description on Env Vars and Config values
• add example to set database.default.encrypt with .env

Checklist:

• Securely signed commits
• [] Component(s) with PHPDoc blocks, only if necessary or adds value
• [] Unit testing, with >80% coverage
• User guide updated
• [] Conforms to style guide

[kenjis]

I have never had a case where the array value needed to be set in an environment variable, but it seems to me that the only way to set SSL settings in MySQL is to set the array in an environment variable.

Therefore, I have added an example of how to set it.

[datamweb]

Interesting description! This explanation will be useful for everyone.

Shouldn't these descriptions be added in the Environment Variables section or related env sections?

[MGatner]

Not thrilled about having a hack in our user guide but it is actually a handy tip.