fontconfig-user.sgml: clarify that version is a fixed-point 16.16 number

https://docs.microsoft.com/en-us/typography/opentype/spec/head says that fontRevision is a "Fixed" number, in particular a 16.16 fixed-point number, representable as a 32-bit integer.

Although the 32-bit integer format is convenient, the documentation should be clear that it's a fixed-point number and explain how to convert version numbers such as 2.008. Here I made a minimal change; I did not check if fontVersion is mentioned in other places.