add a docstring for `Rect` and its aliases #235

Job	Run time
Julia 1.6 - ubuntu-latest - x64 - pull_request	1m 41s
Documentation	1m 59s
Julia 1.6 - macOS-latest - x64 - pull_request	3m 32s
Julia 1.6 - windows-latest - x64 - pull_request	6m 12s
Julia 1 - ubuntu-latest - x64 - pull_request	1m 39s
Julia 1 - macOS-latest - x64 - pull_request	3m 12s
Julia 1 - windows-latest - x64 - pull_request	2m 42s
	20m 57s

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