Add . and .. aliases to ?data.table

[MichaelChirico]

Closes #4407
Closes #4385
Closes #2145 ( #4862 already closed the FAQ element and this PR refines FAQ 1.5 further )

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@5440244). Click here to learn what that means.
The diff coverage is 100.00%.

❗ Current head 1610da7 differs from pull request most recent head e53ad07. Consider uploading reports for the commit e53ad07 to get more accurate results

@@            Coverage Diff            @@
##             master    #4408   +/-   ##
=========================================
  Coverage          ?   99.44%           
=========================================
  Files             ?       73           
  Lines             ?    14469           
  Branches          ?        0           
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  Hits              ?    14388           
  Misses            ?       81           
  Partials          ?        0           

Impacted Files	Coverage Δ
R/data.table.R	99.94% <ø> (ø)
src/assign.c	99.70% <ø> (ø)
src/between.c	99.21% <ø> (ø)
src/bmerge.c	100.00% <ø> (ø)
src/chmatch.c	100.00% <ø> (ø)
src/cj.c	100.00% <ø> (ø)
src/coalesce.c	100.00% <ø> (ø)
src/dogroups.c	99.67% <ø> (ø)
src/fastmean.c	96.82% <ø> (ø)
src/fcast.c	100.00% <ø> (ø)
... and 62 more

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[MichaelChirico]

not sure how I changed coverage 🤔

[avimallu]

I fancied checking earlier open issues, and came across #2145, which is currently partly covered already in ?data.table's examples as:

colNum = 2                     # to refer vars in `j` from the outside of data use `..` prefix
DT[, ..colNum]                 # same, equivalent to DT[, .SD, .SDcols=colNum]

It is also covered in the introduction vignette under the header Select columns named in a variable using the .. prefix.

Lastly, this PR adds the .. operator to the help text, effectively, so I feel that this PR should probably close that issue.