curl_formdata: fix to pass long where missing, document CURLFORM_NAMELENGTH

- lib650: pass `long` to `CURLFORM_NAMELENGTH` in test.
  Spotted by Copilot.
  https://github.com/curl/curl/pull/22011#discussion_r3412407235
  Follow-up to 3620e569b3

- lib650: drop an interim variable, and interim casts.
  Follow-up to 60776a0515 #2747

- curl_formdata.md: document `CURLFORM_NAMELENGTH` on man page.

- curl_formdata.md: pass `long` to `CURLFORM_BUFFERLENGTH` on man page.

- formdata: pass `long` to `CURLFORM_CONTENTSLENGTH` in comment.

Closes #22017

tests/libtest/lib650.c

@@ -48,7 +48,6 @@ static CURLcode test_lib650(const char *URL)
   struct curl_forms formarray[3];
   size_t formlength = 0;
   char flbuf[32];
-  long contentlength = 0;
 
   static const char testname[] = "fieldname";
   static char testdata[] = "this is what we post to the silly web server";
@@ -83,19 +82,17 @@ static CURLcode test_lib650(const char *URL)
     goto test_cleanup;
   }
 
-  contentlength = (long)(strlen(testdata) - 1);
-
   /* Use a form array for the non-copy test. */
   formarray[0].option = CURLFORM_PTRCONTENTS;
   formarray[0].value = testdata;
   formarray[1].option = CURLFORM_CONTENTSLENGTH;
-  formarray[1].value = (char *)(size_t)contentlength;
+  formarray[1].value = (char *)(strlen(testdata) - 1);
   formarray[2].option = CURLFORM_END;
   formarray[2].value = NULL;
   formrc = curl_formadd(&formpost,
                         &lastptr,
                         CURLFORM_PTRNAME, testname,
-                        CURLFORM_NAMELENGTH, sizeof(testname) - 2,
+                        CURLFORM_NAMELENGTH, (long)(sizeof(testname) - 2),
                         CURLFORM_ARRAY, formarray,
                         CURLFORM_FILENAME, "remotefile.txt",
                         CURLFORM_END);