Docs: Improved the docs for writing functions taking Eigen types.
Docs: Improved the docs for writing functions taking Eigen types.

- Removed the wrong statement about the MSVC compiler.
- Reformulated "simple functions" usage.
- Reformulated the summary paragraph about writable parameters.

pasuka/eigen,Zefz/eigen,pasuka/eigen,TSC21/Eigen,Zefz/eigen,TSC21/Eigen,ROCmSoftwarePlatform/hipeigen,ROCmSoftwarePlatform/hipeigen,TSC21/Eigen,TSC21/Eigen,ritsu1228/eigen,toastedcrumpets/eigen,ritsu1228/eigen,ROCmSoftwarePlatform/hipeigen,pasuka/eigen,pasuka/eigen,ritsu1228/eigen,Zefz/eigen,Zefz/eigen,ROCmSoftwarePlatform/hipeigen,toastedcrumpets/eigen,toastedcrumpets/eigen,ritsu1228/eigen,toastedcrumpets/eigen,ritsu1228/eigen,pasuka/eigen