Improve docs for data_to_wide #1731

Job	Run time
R-CMD-check / macOS-latest (release)	1m 56s
R-CMD-check / windows-latest (devel)	1m 27s
R-CMD-check / windows-latest (release)	1m 30s
R-CMD-check / windows-latest (oldrel)	7m 29s
R-CMD-check / windows-latest (4.1)	4m 15s
R-CMD-check / ubuntu-latest (devel)	1m 39s
R-CMD-check / ubuntu-latest (release)	1m 15s
R-CMD-check / ubuntu-latest (4.3)	1m 20s
R-CMD-check / ubuntu-latest (4.2)	1m 14s
R-CMD-check / ubuntu-latest (4.1)	1m 0s
R-CMD-check / ubuntu-latest (4.0)	1m 6s
R-CMD-check / ubuntu-latest (3.6)	1m 0s
	25m 11s

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