docs: Add some clarity to ottrace docs

[arielvalentin]

Fixes #521

[arielvalentin]

@mwear hold on... I committed your changes a little too soon

This propagator is compatible with 8 or 16 byte trace ids which it normalizes to 16 bytes but only emits 8 byte trace ids.

8 bytes (64 bit) or 16 bytes (128 bit) extract but always 8 bytes (64 bit) inject

[mwear]

8 bytes (64 bit) or 16 bytes (128 bit) extract but always 8 bytes (64 bit) inject

☝️ is correct. I'm not sure if we should clarify the inject vs extract case, if you just want to go with what we originally had (before my suggestion).

[arielvalentin]

Suggested change

	The reality of the world is not every application upgraded to support 16 byte array propagation format, but this propagator must still convert legacy 8 byte trace ids to match the W3C Trace Context Trace ID 16 byte array.
	The reality of the world is not every application upgraded to support 16 byte array propagation format, but this propagator must still convert legacy 64 but trace ids to match the W3C Trace Context Trace ID 16 byte array.