Clarify the meaning of "valid" in XR_FB_hand_tracking_aim

According to the docs, the `XR_HAND_TRACKING_AIM_VALID_BIT_FB` in `XrHandTrackingAimFlagsFB` indicates that "Aiming data is valid". But it's ambiguous what valid means in this case: my initial assumption was that invalid data would be garbage and unusable, but while integrating support for this extension in Chromium I found that it actually seems more akin to indicating whether the aim pose passes some threshold of certainty. Just occluding the index and/or thumb is enough to make this bit report 0, but the actual aimPose being returned is (functionally) still usable for the majority of rotations, the main exceptions being when the hand is turned towards the user or at an extreme angle.

Start with the XR_FB_hand_tracking_aim documentation, especially XR_HAND_TRACKING_AIM_VALID_BIT_FB and XrHandTrackingAimFlagsFB, and compare the stated meaning with the observed behavior described here. Done means the specification clearly explains what “valid” means for the aim pose and removes the ambiguity reported during Chromium integration.

Written by the indexing model from the issue text.