Extra { around inherited ifelse tags

[akersting]

Using @inheritParams produces "bad markup" if the Rd inherited from contains \ifelse as e.g. the case for parallel::mclapply.

This is very similar to #778 (fixed in e1d3930). Also here extra {} are added. Hence, adding "\\ifelse" to the vector TWOARG of as_character_rd should fix the issue.

[hadley]

Would you be interested in contributing a PR?