Document the port naming convention of replacing ++ with xx

[blb@…]

Several ports whose "true" name have ++ in their names (libsigc++, libxml++, mysql++, and others) have the plus signs replaced with the letter x (libsigcxx2, libxmlxx, mysqlxx). This is the convention we've picked (and is also used in other project names directly like libpqxx), but we have no current documentation for it in the guide. This should be added someplace.

[jmr@…]

The motivation being of course that the + could be (and probably is) misinterpreted as the start of a variant.