docs: describe what colors in outdated mean

Fixes: #16224
PR-URL: npm/npm#20408
Credit: @teameh
Reviewed-By: @iarna

doc/cli/npm-outdated.md

@@ -27,6 +27,8 @@ In the output:
 * `package type` (when using `--long` / `-l`) tells you whether this package is
   a `dependency` or a `devDependency`. Packages not included in `package.json`
   are always marked `dependencies`.
+* Red means there's a newer version matching your semver requirements, so you should update now.
+* Yellow indicates that there's a newer version above your semver requirements (usually new major, or new 0.x minor) so proceed with caution.
 
 ### An example