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"

PR-URL: #31703
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

Author: Trott

doc/api/assert.md

@@ -57,12 +57,10 @@ assert.deepEqual([[[1, 2, 3]], 4, 5], [[[1, 2, '3']], 4, 5]);
 //   ]
 ```
 
-To deactivate the colors, use the `NO_COLOR` or
-`NODE_DISABLE_COLORS` environment variable.
-This will also deactivate the colors in the REPL.
-
-For more on the color support in terminal environments, read
-the tty [getColorDepth()](tty.html#tty_writestream_getcolordepth_env) doc.
+To deactivate the colors, use the `NO_COLOR` or `NODE_DISABLE_COLORS`
+environment variables. This will also deactivate the colors in the REPL. For
+more on color support in terminal environments, read the tty
+[getColorDepth()](tty.html#tty_writestream_getcolordepth_env) documentation.
 
 ## Legacy assertion mode