Clarify when estimate methods return empty (#2323)

## Description

Clarifies when estimate methods return an empty optional. This also
fixes some parity issues with Constrained SolvePnP not checking the
optional returned by the heading buffer.

Resolves #2322.

## Meta

Merge checklist:
- [x] Pull Request title is [short, imperative
summary](https://cbea.ms/git-commit/) of proposed changes
- [x] The description documents the _what_ and _why_
- [ ] If this PR changes behavior or adds a feature, user documentation
is updated
- [ ] If this PR touches photon-serde, all messages have been
regenerated and hashes have not changed unexpectedly
- [ ] If this PR touches configuration, this is backwards compatible
with settings back to v2025.3.2
- [ ] If this PR touches pipeline settings or anything related to data
exchange, the frontend typing is updated
- [ ] If this PR addresses a bug, a regression test for it is added

docs/source/docs/programming/photonlib/robot-pose-estimator.md

@@ -108,7 +108,7 @@ When taking in a result from a `PhotonCamera`, PhotonPoseEstimator offers nine p
     flat on the floor. This computation takes place on the RoboRIO, and should not take more than 2ms.
     This also requires addHeadingData to be called every frame so heading data is up to date.
 
-Calling one of the `estimate<strategy>Pose()` methods on your `PhotonPoseEstimator` will return an `Optional<EstimatedRobotPose>`, which includes a `Pose3d` of the latest estimated pose (using the selected strategy) along with a `double` of the timestamp when the robot pose was estimated. The recommended way to use the estimatePose methods is to
+Calling one of the `estimate<strategy>Pose()` methods on your `PhotonPoseEstimator` will return an `Optional<EstimatedRobotPose>`, which will be empty if there are no detected tags, not enough detected tags (for multi-tag strategies), missing data (typically heading data), or if the internal solvers failed (this is a rare scenario). `EstimatedRobotPose` includes a `Pose3d` of the latest estimated pose (using the selected strategy) along with a `double` of the timestamp when the robot pose was estimated. The recommended way to use the estimatePose methods is to
 1. do estimation with one of MultiTag methods, check if the result is empty, then
 2. fallback to single tag estimation using a method like `estimateLowestAmbiguityPose`.