Lack of Doc Comments in some Render Related Files

## How can Bevy's documentation be improved?

There seems to be a general lack of documentation comments in certain render related files, e.g.
[pipeline_specializer](https://docs.rs/bevy_render/0.13.2/src/bevy_render/render_resource/pipeline_specializer.rs.html#53),
[mesh](https://docs.rs/bevy_sprite/0.13.2/src/bevy_sprite/mesh2d/mesh.rs.html#242).

Items such as Specialized
[Compute](https://docs.rs/bevy/latest/bevy/render/render_resource/struct.SpecializedComputePipelines.html), [Mesh](https://docs.rs/bevy/latest/bevy/render/render_resource/struct.SpecializedMeshPipelines.html), and [Render](https://docs.rs/bevy/latest/bevy/render/render_resource/struct.SpecializedRenderPipelines.html) Pipelines, are generated with barebones documentation. As a result it becomes more difficult to figure their purpose in examples which feature them, discern differences between similar structs (such as the three pipelines linked above), or glean hints about how rendering works in general from studying the code base.

I'm not able to suggest documentation for these code since I don't understand it or rendering very well.

Also I know there's a bunch of cool render-related features in the works. I don't mind if I have to wait for those to drop.

Start by reading the linked pipeline_specializer.rs and mesh.rs files, then compare the documentation for SpecializedComputePipelines, SpecializedMeshPipelines, and SpecializedRenderPipelines. Define the missing explanations for their purposes and differences, add focused Rust documentation comments, and verify that the generated documentation clearly explains their use in rendering examples.

Written by the indexing model from the issue text.