Missing public/exported docstrings

Found using `Docs.undocumented_names` from 1.11.
They should also be added to the documentation when a docstring is created.

### Metal
- [X] `Metal`
- [X] `is_managed`
- [X] `is_private`
- [X] `is_shared`
- [X] `device`
- [X] `MtlMatrix`
- [X] `MtlVecOrMat`
- [X] `MtlVector`
- [X] `MetalBackend`
- [X] `MtlDeviceMatrix`
- [X] `MtlDeviceVector`
- [X] `MTL`
- [X] `MPS`

### MTL
Important:
- [X] `Managed`
- [X] `Private`
- [X] `Shared`
- [X] `CPUStorage`

Other:
- [ ] `air_support`
- [ ] `air_version`
- [ ] `is_m1`
- [ ] `is_m2`
- [ ] `is_m3`
- [ ] `macos_version`
- [ ] `metal_support`
- [ ] `metal_version`
- [ ] `metallib_support`
- [ ] `darwin_version`
- [ ] `supports_family`
- [ ] `add_functions!`
- [ ] `append_current_function!`
- [ ] `commit!`
- [ ] `dispatchThreadgroups!`
- [ ] `endEncoding!`
- [ ] `set_buffer!`
- [ ] `set_function!`
- [ ] `supports_destination`
- [ ] `MTLBinaryArchive`
- [ ] `MTLBinaryArchiveDescriptor`
- [ ] `MTLBuffer`
- [ ] `MTLCompileOptions`
- [ ] `MTLComputeCommandEncoder`
- [ ] `MTLComputePipelineDescriptor`
- [ ] `MTLComputePipelineState`
- [ ] `MTLCreateSystemDefaultDevice`
- [ ] `MTLEvent`
- [ ] `MTLFence`
- [ ] `MTLFunction`
- [ ] `MTLFunctionDescriptor`
- [ ] `MTLHeap`
- [ ] `MTLHeapDescriptor`
- [ ] `MTLLibrary`
- [ ] `MTLLibraryFromData`
- [ ] `MTLLibraryFromFile`
- [ ] `MTLOrigin`
- [ ] `MTLRegion`
- [ ] `MTLSharedEvent`
- [ ] `MTLSharedEventHandle`
- [ ] `MTLSize`
- [ ] `MTLTexture`
- [ ] `MTLTextureDescriptor`
- [ ] `MTLCommandBuffer`
- [ ] `MTLCommandBufferDescriptor`
- [ ] `MTLCommandQueue`
- [ ] `ReadUsage`
- [ ] `ReadWriteUsage`
- [ ] `WriteUsage`
- [ ] `MTLResource`
- [ ] `MTLBlitCommandEncoder`
- [ ] `append_copy!`
- [ ] `append_fillbuffer!`
- [ ] `append_sync!`

Unexport:
- [x] `@sv_str`
- [x] `SimpleVersion`
- [x] `AS`