[Rd] Warnings about unrecognized escapes in Rd

[Seth Falcon]

Hi,

The warning that was recently added to R-devel about unrecognized
escapes has been very helpful in identifying a number of incorrectly
specified regular expressions.

We are, however, seeing warning messages for Rd files that contiain
'\_' and I think that _is_ a recognized escape.

Can the check be refined so that '\_' is not warned about in Rd files?

+ seth