docs: Improve String concatenation best practice

[ppkarwasz]

This change splits the "don't use String concatenation" best practice into two parts:

• A recommendation concerning performance: string concatenation is not efficient if the logger is off.
• A security recommendation: format string must be constants to prevent {} placeholder injection.

The ${dangerousLookup} part of the example is removed, since lookups are not executed since version 2.15.0