Add better documentation on how id_function() should be defined and what

issues there are.

PR: 1096

doc/crypto/threads.pod

@@ -65,18 +65,10 @@ B<CRYPTO_LOCK>, and releases it otherwise.
 B<file> and B<line> are the file number of the function setting the
 lock. They can be useful for debugging.
 
-id_function(void) is a function that returns a thread ID, for
-instance, pthread_self(). It is not, needed on Windows nor on
-platforms where getpid() returns a different ID for each thread.
-However, even on those platforms, pthread_self() should be used, since
-the behavior of getpid() may depend on the machine where the program
-is being run, not the machine where the program is being compiled.
-(For instance, Red Hat 8 Linux and earlier used LinuxThreads, whose
-getpid() returns a different value for each thread; Red Hat 9 Linux
-and later use NPTL, which is Posix-conformant, and thus whose getpid()
-returns the same value for all threads in a process.  But a program
-compiled on Red Hat 8 and run on Red Hat 9 will by default see
-getpid() returning the same value for all threads.)
+id_function(void) is a function that returns a thread ID, for example
+pthread_self() if it returns an integer (see NOTES below).  It isn't
+needed on Windows nor on platforms where getpid() returns a different
+ID for each thread (see NOTES below).
 
 Additionally, OpenSSL supports dynamic locks, and sometimes, some parts
 of OpenSSL need it for better performance.  To enable this, the following
@@ -133,7 +125,7 @@ CRYPTO_get_new_dynlockid() returns the index to the newly created lock.
 
 The other functions return no values.
 
-=head1 NOTE
+=head1 NOTES
 
 You can find out if OpenSSL was configured with thread support:
 
@@ -148,6 +140,22 @@ You can find out if OpenSSL was configured with thread support:
 Also, dynamic locks are currently not used internally by OpenSSL, but
 may do so in the future.
 
+Defining id_function(void) has it's own issues.  Generally speaking,
+pthread_self() should be used, even on platforms where getpid() gives
+different answers in each thread, since that may depend on the machine
+the program is run on, not the machine where the program is being
+compiled.  For instance, Red Hat 8 Linux and earlier used
+LinuxThreads, whose getpid() returns a different value for each
+thread.  Red Hat 9 Linux and later use NPTL, which is
+Posix-conformant, and has a getpid() that returns the same value for
+all threads in a process.  A program compiled on Red Hat 8 and run on
+Red Hat 9 will therefore see getpid() returning the same value for
+all threads.
+
+There is still the issue of platforms where pthread_self() returns
+something other than an integer.  This is a bit unusual, and this
+manual has no cookbook solution for that case.
+
 =head1 EXAMPLES
 
 B<crypto/threads/mttest.c> shows examples of the callback functions on