Docs: Removed duplicated content in Marker docs

[f1ames]

Suggested merge commit message (convention)

Docs: Removed duplicated content in Marker docs.

Additional information

There is one duplicated section in Marker class docs:

ckeditor5-engine/src/model/markercollection.js

Lines 257 to 259 in fd2005d

	* Another upside of markers is that finding marked part of document is fast and easy. Using attributes to mark some nodes
	* and then trying to find that part of document would require traversing whole document tree. Marker gives instant access
	* to the range which it is marking at the moment.

ckeditor5-engine/src/model/markercollection.js

Lines 308 to 310 in fd2005d

	* Another upside of markers is that finding marked part of document is fast and easy. Using attributes to mark some nodes
	* and then trying to find that part of document would require traversing whole document tree. Marker gives instant access
	* to the range which it is marking at the moment.

[coveralls]

Coverage remained the same at 100.0% when pulling c0a072f on t/docs-markercollection into fd2005d on master.