Fix calibration tutorial docs, decomposeProjectionMatrix, and convertMaps performance claims #29487Fixes#25655, #26791, #27277.
Three doc fixes:
1. Calibration tutorial: rows/cols swapped, fixed np.mgrid consistency
2. decomposeProjectionMatrix: clarified transVect is camera center in homogeneous coordinates
3. convertMaps: replaced overstated 2x speed claim
### Pull Request Readiness Checklist
- [x] I agree to contribute under Apache 2 License
- [x] Not based on GPL/incompatible license
- [x] PR proposed to proper branch (4.x)
- [x] Reference to original bug report and related work
- [ ] Accuracy test, performance test, test data: N/A (doc-only)
- [x] Feature well documented and sample code buildable