Add documentation to general.yml parameters

Also available at http://alaveteli.org/docs/customising/config
Add default INCOMING_EMAIL_PREFIX

All our examples use the 'foi+' prefix so lets make this the default. If
people _really_ want no prefix, they can change the setting to ''. This
will probably cover the more general cases.
Emphasise suggestion to change emergency admin details
Don't set non-default value for USE_GHOSTSCRIPT_COMPRESSION

We don't set this to true as a default so don't supply the non-default
to new installs.
Set default MAX_REQUESTS_PER_USER_PER_DAY

We supply a default example for MAX_REQUESTS_PER_USER_PER_DAY so we
might as well set it as default and allow the user to tune if they
dislike the default.
Add default MTA_LOG_PATH

We default MTA_LOG_TYPE to exim so we can set the default log path too
Use default setting for MINIMUM_REQUESTS_FOR_STATISTICS

Defaults to 100 so use this in the example config
Set placeholder config to boolean opposites

If we're setting the placeholder to the default value, comment it out
and set to the inverse, so that to change the setting you just uncomment
the line.