Documentation Typos and Grammar Improvements

Just keeping a running list of documentation typos so that they can be fixed in future releases. The correction is provided after the dash.

Unity Plugin

[ ] Installation instructions: “which can be found in the specific platfomr archive” - platform
[ ] The position and velocity of joints is cached - are cached
[ ] In addition, Unity also needs the native MuJoCo library, which can be found in the specific platfomr archive at https://github.com/deepmind/mujoco/release. The URL is a 404, it should be https://github.com/deepmind/mujoco/releases

[ ] Very minor nit, but can we add a newline between every Defined in *.h and the explanation right after?
[ ] API functions (and letting the compiler do the enum-to-int type cast.) - period after parentheses
[ ] Collision modes specifying how candidate geom pair are generated for near-phase collision checking - geom pairs
[ ] For the 3D mouse (SpaceNavigator) both can be enabled simultaneously. Tehy are used in mjvPerturb.active - they
[ ] All user callbacks (i.e. global function pointers whose name starts with ‘mjcb’) - i.e.,
[ ] When installed, this function overrides the default error processing. Once it prints error mesages - messages

[ ] Once a high-level mjCModel is created - by loading an MJCF file or an URDF file - a URDF file
[ ] The body specifies childclass “sub”, causing all children of this body (and all their children etc) to use class “sub” unless specified otherwise - etc.
[ ] A final twist here are actuators - is actuators
[ ] For meshes, the 3D vertex positions are expressed in either local and global coordinates - either local or global
[ ] See the Modular Prosthetic Limb (MPL) hand model available on the MuJoCo Forum for an example; the plots below are generated with that model. - Is the MuJoCo Forum always gonna be available? And unsure if the model files are available?

[ ] Note that the profiler shows timing information collected with high-resolution timers. On Windows, depending on the power settings, the OS may reduce the CPU frequency; this is because simulate.cc - simulate.cc should link the source code on GitHub.
[ ] The rendering is simplified compared to simulate.cc because there is no user interaction, visualization options or timing - same as above
[ ] This is not a stand-alone code sample, but rather a small utility used to hook up the new UI to GLFW. It is used in simulate.cc and can also - same as above