[doc] Fix HTML in platform labels, add tooltip
[doc] Fix HTML in platform labels, add tooltip

Fix the opening <span> tag that mistakenly ends with '/>'.

Add tooltips to clarify the meaning of the platform tags.

Change-Id: Ia3d2862ec8262f0ed5f5cecefe96b427ca215972

eclipsesource/tabris-js,eclipsesource/tabris-js,eclipsesource/tabris-js