Enhance help files for draw_figure_label and draw_label

[tbates]

I think these edits might help people pick the functions they need (for instance, make draw_figure_label, more discoverable by putting its purpose first in description).

Also aid understanding of complex things like hjust and coordinates, by giving examples with labeled parameters, labels which say where they were placed, and more help text among examples.

Finally, more fulsome labels for @params to help people understand them.

[clauswilke]

Thanks for your contribution. A couple of requests before I merge this:

• Please use US spelling, not British. ("centered" not "centred", lines 69 & 70.)
• Please make sure there's a space after every comma and around every equals sign (e.g. lines 70, 86, 103, 104, 105).
• Please write 0.5 instead of .5 etc. (many instances).
• I find the non-exported comments in lines 96-98 strange. Please remove.

Edit: Line numbers seem to have changed since I wrote this.

[codecov-io]

Codecov Report

Merging #76 into master will decrease coverage by 0.08%. The diff coverage is 42.85%.

@@            Coverage Diff             @@
##           master      #76      +/-   ##
==========================================
- Coverage   33.77%   33.68%   -0.09%     
==========================================
  Files          13       13              
  Lines         749      751       +2     
==========================================
  Hits          253      253              
- Misses        496      498       +2

Impacted Files	Coverage Δ
R/draw.R	10.22% <42.85%> (-0.24%)	:arrow_down:

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[clauswilke]

In your updated patch, please delete "(yet)" from "(not (yet) expressions)". Regardless of the merits of your proposed merge of draw_text and draw_label, this comment does not belong into the documentation.

[tbates]

I've formatted ", " and " = "; moved to US spelling, prefaced decimals with a 0 (also did this reformattin in couple of other places in the code for consistency). The un-exported comment is deleted: it was supposed to be an inline block-comment to help readers parse the help (but I didn't type the requisite #' in front). Gone now. Sorry for the string of updates instead of one-clean unit patch... won't happen again.

#' # ===================================================
#' # = Add labels via ggdraw. Uses ggdraw coordinates. =
#' # ===================================================