doc: clarify how config files are configured

PR-URL: npm/npm#13911
Credit: @othiym23
Reviewed-By: @zkat

doc/misc/npm-config.md

@@ -26,10 +26,12 @@ same.
 
 The four relevant files are:
 
-* per-project config file (/path/to/my/project/.npmrc)
-* per-user config file (${NPM_CONFIG_USERCONFIG:-~/.npmrc})
-* global config file ($PREFIX/etc/npmrc)
-* npm builtin config file (/path/to/npm/npmrc)
+* per-project configuration file (`/path/to/my/project/.npmrc`)
+* per-user configuration file (defaults to `$HOME/.npmrc`; configurable via CLI
+  option `--userconfig` or environment variable `$NPM_CONF_USERCONFIG`)
+* global configuration file (defaults to `$PREFIX/etc/npmrc`; configurable via
+  CLI option `--globalconfig` or environment variable `$NPM_CONF_GLOBALCONFIG`)
+* npm's built-in configuration file (`/path/to/npm/npmrc`)
 
 See npmrc(5) for more details.