doc: improve strict assertion mode color text
Minor improvements to the text about colors in strict assertion mode:
- Consolidate the two paragraphs into one
- Pluralize "variables"
- Remove unnecessary article "the"
- Change "doc" to "documentation"
Checklist
-
make -j4 test
(UNIX), orvcbuild test
(Windows) passes -
documentation is changed or added -
commit message follows commit guidelines