fix getDocumentPointsize to work with "a4paper" documentclass option

[corecode]

The previous regexp would match any number that was followed by either
"p" or "t" and use this as the document point size. This led to

\documentclass[a4paper]{artice}

being interpreted as a 4pt document. As a result, plotted figures
wound up with grossly scaled font nodes (scale=3 or similar).

We instead want to only match digits at the beginning of a word,
followed by the string "pt".

[yihui]

This is not a complete fix. I think you should use '\\<\\d+pt\\>' to avoid cases like 12ptabc. Accordingly there should be a change later in this function: increase 2 to 3 because pt has two characters

pointsize <- substr(docString, psLocation, psLocation + 
            attr(psLocation, "match.length") - 3)

[corecode]

Thanks, I fixed accordingly.