Add note about UCP in PCRE.

git-svn-id: https://svn.r-project.org/R/trunk@84799 00db46b3-68df-0310-9c12-caf00c1e9a41

src/library/base/man/regex.Rd

@@ -309,7 +309,9 @@
  \samp{\p{xx}} and \samp{\P{xx}} which match characters with and
  without property \samp{xx} respectively. For a list of supported
  properties see the PCRE documentation, but for example \samp{Lu} is
- \sQuote{upper case letter} and \samp{Sc} is \sQuote{currency symbol}.
+ \sQuote{upper case letter} and \samp{Sc} is \sQuote{currency symbol}. Note
+ that for some properties to work, one may have to start the pattern with
+ \samp{(*UCP)} and accept a performance penalty.
  (This support depends on the PCRE library being compiled with
  \sQuote{Unicode property support} which can be checked \emph{via}
  \code{\link{pcre_config}}. PCRE2 when compiled with Unicode support always