[docs] Better document the ref props in the API

[oliviertassinari]

Closes #17136

[mui-pr-bot]

Details of bundle changes.

Comparing: 8ec0e6f...4bf64db

bundle	parsed diff	gzip diff	prev parsed	current parsed	prev gzip	current gzip
@material-ui/core	0.00%	0.00%	329,524	329,524	90,000	90,002
@material-ui/core/Paper	0.00%	-0.01%	68,774	68,774	20,487	20,485
@material-ui/core/Paper.esm	0.00%	+0.01% 🔺	62,148	62,148	19,215	19,216
@material-ui/core/Popper	0.00%	0.00%	28,468	28,468	10,177	10,177
@material-ui/core/Textarea	0.00%	0.00%	5,094	5,094	2,137	2,137
@material-ui/core/TrapFocus	0.00%	-0.06%	3,834	3,834	1,614	1,613
@material-ui/core/styles/createMuiTheme	0.00%	0.00%	16,386	16,386	5,827	5,827
@material-ui/core/useMediaQuery	0.00%	0.00%	2,541	2,541	1,058	1,058
@material-ui/lab	0.00%	-0.00%	153,304	153,304	46,710	46,709
@material-ui/styles	0.00%	-0.01%	51,492	51,492	15,305	15,304
@material-ui/system	0.00%	-0.11%	15,658	15,658	4,371	4,366
Button	0.00%	0.00%	78,778	78,778	24,073	24,074
Modal	0.00%	+0.02% 🔺	14,346	14,346	5,009	5,010
Portal	0.00%	-0.08%	2,907	2,907	1,319	1,318
Rating	0.00%	+0.01% 🔺	70,245	70,245	21,942	21,945
Slider	0.00%	-0.00%	74,483	74,483	23,065	23,064
colorManipulator	0.00%	0.00%	3,904	3,904	1,543	1,543
docs.landing	0.00%	0.00%	52,305	52,305	13,790	13,790
docs.main	0.00%	0.00%	597,298	597,298	190,849	190,849
packages/material-ui/build/umd/material-ui.production.min.js	0.00%	0.00%	300,429	300,429	86,412	86,412

Generated by 🚫 dangerJS against 4bf64db