[O] Doc suggestion: Clarify subtree export properties

[James Harkins]

I think the manual could be a bit clearer on one point of the use of :EXPORT_*: 
properties.

http://orgmode.org/manual/Export-settings.html#Export-settings

~~
When exporting only a subtree, each of the previous keywords3 can be overridden 
locally by special node properties. These begin with ‘EXPORT_’, followed by the 
name of the keyword they supplant. For example, ‘DATE’ and ‘OPTIONS’ keywords 
become, respectively, ‘EXPORT_DATE’ and ‘EXPORT_OPTIONS’ properties.
~~

And...

~~
[2] If you want to configure many options this way, you can use several 
#+OPTIONS lines.
~~

*If* you already know what you're doing, then you know that you can't have 
multiple properties of the same name in the same PROPERTIES drawer.

If you are reading the manual because you don't already know what you're doing, 
then you might simply convert the default block of options (inserted by C-c C-e 
# default RET) into multiple lines of properties, and then get annoyed when 
many of them are ignored.

Perhaps add this?

~~
... become, respectively, ‘EXPORT_DATE’ and ‘EXPORT_OPTIONS’ properties. Note 
that multiple EXPORT_OPTIONS lines are not supported, because node properties 
must have unique identifiers.
~~

hjh