The suggested examples are misleading because they use 6.24 which is not exactly representable in binary floating point.  Representation issues are orthogonal to the OP's issue which is really just a simple rounding example:

>>> x = float.fromhex('0x0.1234560000001')
>>> unpack('!f', pack('!f', x))[0].hex()
'0x1.2345600000000p-4'

Also, if something like the suggested note is adopted, it needs to be worded in a way that doesn't imply that the struct implementation is broken or misdesigned.  

A better note would focus on the basic (and obvious) fact that downgrading from double precision to single precision  
entails a loss of precision.