XML: Added a note that string sources are not modified.
XML: Added a note that string sources are not modified.

Update issue 1690
Summary: XML library: Document that sources given as strings are not modified in place
Status: Done
Owner: pekka.klarck
Labels: Priority-Low
The same text about modifying the source was in all keywords that
modify XML. Added a note that strings themselves are not modified to
`Get Element` keyword that those keywords use internally and link from
their docs.

userzimmermann/robotframework-python3,userzimmermann/robotframework-python3,userzimmermann/robotframework-python3,userzimmermann/robotframework-python3,userzimmermann/robotframework-python3