documentation: add info about cmdargs (#1862)

Co-authored-by: Zeid Zabaneh <[email protected]>

docs/documentation/configuration.md

@@ -19,6 +19,13 @@ Location:
 Each option correspond to its command-line counter part (the long form,
 but without the first double dashes, -\-). They are described [here](usage.html).
 
+There is an exception for `--arg` which can be set in `mozregression.cfg` by using `cmdargs`.
+For example, to pass a URL at which Firefox will open use
+```
+cmdargs = "https://mozilla.org",
+```
+Note the trailing comma, it is required to be correctly parsed as a list.
+
 ## Example
 
 {% highlight ini %}

docs/documentation/usage.md

@@ -52,7 +52,7 @@ to date list of available options.
         string.pref = Hello, world!
         integer.pref = 1
 
-- Forward arguments to tested binary (note that you must use '=' for arguments beginning with '-' so that the     parser doesn't get confused)
+- Forward arguments to tested binary. Note that you must use '=' for arguments beginning with '-' so that the parser doesn't get confused. Additionally, this is equivalent to `cmdargs` in the [configuration file](configuration.html).
 
         mozregression --arg='-foreground' --arg='-P'