Clarify documentation for inlining JS

##### Changes:

- Lowercase the title (to `emscripten.h`)
- Add `EM_ASM_ARGS` to suggestions for I/O functions (L38)
- Separated documentation for I/O components (L50-63)

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- Does __not__ remove this note (L23), though it appears outdated. @hamishwillee , should I remove this in an amended commit?

  > .. todo:: **HamishW** This should be linked to section "Calling JavaScript From C/C++" when it exists.

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_This affects the `Emscripten.h` documentation page, marked `ready-for-review`._