docs: change RealField(64) to ArbField(64) or RealField()

[fingolfin]

There are still plenty of examples in the manual which use RealField(64) or ComplexField(64).

Should these be changed to use RealField(), or to use ArbField(64) ? I guess it depends on what the long term goal is: keep both? Migrate all to the "new" RealField? Something else?

[lgoettgens]

$ grep "RealField(64)" -r . | wc -l
0
$ grep "ComplexField(64)" -r . | wc -l
0

According to my bash, there are no instances left to change (on f033978)