Add documented unit tests for formatValue()

Add example showing how to forward format specifiers.
Add example highlighting the similarity with formattedWrite(), and
comparing their use.

Change /** */ for /// and add $(D formatValue)

Replace /** */ with /++ +/
std.algorithm.remove: Fix constraints and change popBack loop to popBackN
std.algorithm.remove: Change pop____N to pop____Exactly
Merge pull request #2256 from Safety0ff/popbackn

std.algorithm.remove: Fix constraints and change popBack loop to popBackN
Merge pull request #2296 from Element-126/doc_formatValue_add_example

Add documentation examples for formatValue()