Note for external storage types that use temp space

**Do not merge before https://github.com/owncloud/core/pull/13031**
**If the above PR gets merged, this one should be merged to finalize the documentation**

This PR documents that some external storage types may need a higher temporary disk space due to lack of steaming capabilities and references to a parameter to configure the temp dir location which is defined in core 13031

The note is written in configuration/external_storage_configuration gui and non gui version.

The text looks good, I have a couple of formatting tweaks: ``tempdirectory`` and ``config.sample.php`` should be enclosed in double-backticks. (Someday I will update the style guide.)