bug#10185: 24.0.50; rgrep breaks when called programmatically

[Juri Linkov]

Version: 24.0.92

>> Evaluating (rgrep "hello") results in "/bin/bash: hello: command not found"
>>
>> The docstring for rgrep claims that its first argument is intended to be
>> a regexp, but clearly it's attempting to execute it via bash.
>
> `rgrep' assumes that when the `FILES' argument is empty then a grep command
> is specified in the `REGEXP' argument.  This should be documented.

Docstring fixed.  Thanks for the report.