[Rd] disappearing paragraphs in html rendering of text from Rd macros

[Georgi Boshnakov]

Recent changes in R-devel seem to change the HTML rendering of text returned by Rd macros,
in that now empty lines do not cause paragraphs to end (the empty lines are kept but <p> tags are not set).
As a result, multiple paragraphs are output as a single one.

A complete example Rd file is at: https://github.com/GeoBosh/reprexes/blob/master/disappearingParagraphs/a.Rd

rendered (2018-10-28) with R-devel: https://github.com/GeoBosh/reprexes/blob/master/disappearingParagraphs/a.html 

and R-3.5.1patched: https://github.com/GeoBosh/reprexes/blob/master/disappearingParagraphs/a351.html 

The exact system information used for rendering is in the above html files. 

In essence, the following:

\newcommand{\abc}{\Sexpr[stage=build,results=rd]{paste0(letters[1:4], collapse = "\n\n")}}

produces in R-devel something like 

<p>a

b

c

d</p>

where the empty lines may be distorted in this email, while in R-3.5.1 the empty lines cause paragraph tags around each chunk:

<p>a</p>

<p>b</p>

<p>c</p>

<p>d</p>

Directly embedding the Sexpr in the Rd file also produces all <p> tags in R-devel and R-3.5.1patched.,

---

Georgi Boshnakov