DOC: clarify OPENSSL_API_COMPAT

Fixes #15928

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15956)

doc/man7/openssl_user_macros.pod.in

@@ -74,16 +74,18 @@ minor and patch components of the version number.  For example:
 
 =back
 
-If not set, this macro will default to
+If B<OPENSSL_API_COMPAT> is undefined, this default value is used in its
+place:
 C<{- join('', map { my @x = split /=/,$_; $x[1] }
               grep /^OPENSSL_CONFIGURED_API=/, @{$config{openssl_api_defines} // []})
      || '0x00000000L'
-  -}>.
+  -}>
 
 =item B<OPENSSL_NO_DEPRECATED>
 
 If this macro is defined, all deprecated public symbols in all OpenSSL
 versions up to and including the version given by B<OPENSSL_API_COMPAT>
+(or the default value given above, when B<OPENSSL_API_COMPAT> isn't defined)
 will be hidden.
 
 =back