added backticks in some docstring

[jpcaveiro]

Added backticks for readability in some cases like:

        """The abstract of a document.
        Note: If this is empty, try `description` property instead.
        """

[Michael-E-Rose]

Thanks! While we're at this, let's harmonise the docstrings already. Sometimes I wrote "property XYZ", sometimes I wrote ".XYZ". Let's drop the dot notation and always use "XYZ property".