Docs: Clarify the description for get_temp_dir().

Includes a note that `sys_get_temp_dir()` honors the `TMPDIR` environment variable.

Follow-up to [17555], [22008], [28936].

Props TimoTijhof, SergeyBiryukov.
See #63711.

git-svn-id: https://develop.svn.wordpress.org/trunk@60776 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/functions.php

@@ -2209,12 +2209,14 @@ function wp_normalize_path( $path ) {
 /**
  * Determines a writable directory for temporary files.
  *
- * Function's preference is the return value of sys_get_temp_dir(),
- * followed by your PHP temporary upload directory, followed by WP_CONTENT_DIR,
- * before finally defaulting to /tmp/
+ * Function's preference is the return value of `sys_get_temp_dir()`,
+ * followed by the `upload_tmp_dir` value from `php.ini`, followed by `WP_CONTENT_DIR`,
+ * before finally defaulting to `/tmp/`.
+ *
+ * Note that `sys_get_temp_dir()` honors the `TMPDIR` environment variable.
  *
  * In the event that this function does not find a writable location,
- * It may be overridden by the WP_TEMP_DIR constant in your wp-config.php file.
+ * it may be overridden by the `WP_TEMP_DIR` constant in your `wp-config.php` file.
  *
  * @since 2.5.0
  *