repeat the warning on R API in threads in the R API chapter

git-svn-id: https://svn.r-project.org/R/trunk@88194 00db46b3-68df-0310-9c12-caf00c1e9a41

Author: ripley

doc/manual/R-exts.texi

@@ -14881,6 +14881,10 @@ can be called from C code (and a few that can be called from Fortran
 code).  Only those documented here are stable enough that they will only
 be changed with considerable notice.
 
+As explained elsewhere in this manual, these functions should only be
+called from the main thread of the @R{} process.  (Doing otherwise can
+result in memory corruption and very hard-to-debug seqfaults.)
+
 The recommended procedure to use these is to include the header file
 @file{R.h} in your C code by