Fix javadoc warnings (#2266)

Persuant to #1093, I added as many docstrings as I could, at least for
things I knew about. Some of the classes I just suppressed the Javadoc
warnings in because they aren't particularly useful to document. This
gets us down to less than 100 Javadoc warnings in total. Docs for core
classes on the C++ side were also added for parity.

photon-targeting/src/main/native/include/photon/targeting/PhotonPipelineResult.h

@@ -94,7 +94,10 @@ class PhotonPipelineResult : public PhotonPipelineResult_PhotonStruct {
   }
 
   /**
-   * Returns the estimated time the frame was taken,
+   * Returns the estimated time the frame was taken, in the Time Sync Server's
+   * time base (nt::Now). This is calculated using the estimated offset between
+   * Time Sync Server time and local time. The robot shall run a server, so the
+   * offset shall be 0.
    * This is much more accurate than using GetLatency()
    * @return The timestamp in seconds or -1 if this result was not initiated
    * with a timestamp.
@@ -105,8 +108,11 @@ class PhotonPipelineResult : public PhotonPipelineResult_PhotonStruct {
 
   /**
    * Return the latest mulit-target result, as calculated on your coprocessor.
-   * Be sure to check getMultiTagResult().estimatedPose.isPresent before using
+   * Be sure to check `MultiTagResult().has_value()` before using
    * the pose estimate!
+   *
+   * @return The multi-target result. Empty if there's no multi-target
+   * result/Multi-Target Estimation is disabled in the UI.
    */
   const std::optional<MultiTargetPNPResult>& MultiTagResult() const {
     return multitagResult;