Hello!

Proposing to add some useful information into "Receiving emails" section of Redmine documentation, this information contains:

1) Email body keyword(in docs presents only "assigned_to" override option) "Assigned To". Very useful keyword but currently there is no such description and user must guess from all possible variants like "assigned to" "assigned_to" "Assigned to" etc...

2) No date format for keywords "Start date" and "Due date". The current format is YYYY-MM-DD, despite documentation says:

The values available are the ones of the context. E.g. Status available (for this Tracker and this
 Project) are labels in the localized language, exactly as displayed in the user interface or
 system default language (even with spaces, without quoting).

In my Redmine format for dates in interface is DD.MM.YYYY and it obviously not working. Only way now to get this information is search on redmine site that actually gives you the note from contributor in some ticket years ago.

3) Warning about how important to really know that demanding values for keywords are actually allowed by Redmine, because user won't see an error of this kind from production log. Mail_handler just successfully creates an issue with parameters that allowed by Redmine and ignores email body parameters that are not allowed silently.

For example if my redmine workflow doesn't permits to change assignee while ticket having status "deferred" and if i send email with body like:

Status: Deferred
Assigned To: Eduard Gan
Start date: 2015.09.25

I'll just get the ticket with no assignee and no information from mail_handler about why is that in production log.

Thanks!