If I understand correctly, this doc patch would apply to 2.7 and 3.2 also. I have two style comments. I believe

"It is important to note that bit field allocation and layout in memory is not defined as a standard, rather its implementation is compiler-specific." 

could be shortened to

"Bit field allocation and memory layout is compiler-specific."

To me, this leads nicely into the proposed sentence that follows.

"it is recommended that no assumptions are made about the structure size and layout."

I do not like 'it is recommended'. Let us state the fact.

"any assumptions about the structure size and layout may be wrong."