Update documentation of bitmask types that they does not have to be always prefixed

[mikir]

Discussed in #617

^{Originally posted by Danaozhong May 19, 2024}
The zserio specification about bitmasks states the following:

In expressions outside of the defining type, bitmask values must always be prefixed by the type name and a dot, e.g. Permission.WRITABLE.

This doesn't seem to apply to the isset() operator, as the example provided shows that writing the following is possible:
return isset(testBitmask, INT);
This example also compiles with zserio (tested version 2.11.0).
This code does not compile:
return ((testBitmask & INT) == TestBitmask.INT);

The isset() example is not within the defining type, but omits the type name and dot. Based on the documentation of bitmask types, I would have expected only the following syntax to be legal:
return isset(testBitmask, TestBitmask.INT);

That feels a bit inconsistent to me. Of course, it makes sense that isset() can deduce the bitmask type from the first operand, but it seems to contradict the documentation.