Minor cleanup.
Doc++
Added MAGNUM_TARGET_WEBGL definition.

Treating JavaScript and WebGL as first-class compile target. What the
world had come to :-)
doc: document all WebGL restrictions.
Use more general MAGNUM_TARGET_WEBGL instead of CORRADE_TARGET_EMSCRIPTEN.

Only where it matters, though.
Platform: show MAGNUM_TARGET_WEBGL in magnum-info.