mono-docs: Add <value> tags to properties

mono-docs: Add <value> tags to properties

eolian_mono now generates properties (which simply wrap the setter and getter
methods when both ara available), but they were missing docs, because
properties require a special <value> tag instead of <returns> or <param> which
we are already implementing.
This commit adds <value> tags only if docs can be retrieved from the setter or
the getter methods.