doc: clarify that CLI invocation must always be valid
doc: clarify that CLI invocation must always be valid

This comes up as a corner case where folks provide -e/--regexp in a
configuration file and then expect to be able to run 'rg' with no args.
However, ripgrep fails because it still expects at least one pattern
even though one was specified in the config file.

This occurs because ripgrep has to parse its CLI parameters before
reading the config file. (For log output settings and to handle the
--no-config flag.) This initial parse will fail if there are no patterns
specified.

The only way to solve this that I can see is to somehow relax the
requirements of the initial parse. But this is problematic because we
would still need to enforce those requirements in cases where we don't
do a second parse (when no config file is present).

All in all, this doesn't seem like a problem that is worth solving.

Closes #1730

BurntSushi/ripgrep,BurntSushi/ripgrep,BurntSushi/ripgrep,BurntSushi/ripgrep