End part of the first paragraph in "Details" section:
If the arguments are vectors, they are concatenated term-by-term to give a character vector result. Vector arguments are recycled as needed, with zero-length arguments being recycled to '""' only if 'recycle0' is not true _or_ 'collapse' is not 'NULL'.
I suggest removing " _or_ 'collapse' is not 'NULL'".
'collapse' has nothing to do with recycling.
Description of 'recycle0' in "Arguments" section:
'logical' indicating if zero-length character arguments should lead to the zero-length 'character(0)' after the 'sep'-phase (which turns into '""' in the 'collapse'-phase, i.e., when 'collapse' is not 'NULL')
I suggest the explanation to include the term "recycle" or "recycling" like the argument name. For example,
'logical' indicating if "zero-length recycling rule" is in effect, where mix of zero-length and nonzero-length arguments leads to zero-length ...
... where, in mix of zero-length and nonzero-length arguments, zero-length wins and the nonzero-length arguments are truncated to zero-length