Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Closes https://github.com/gitlabhq/gitlabhq/issues/9328

capitalized.

Make the plural version consistently "URLs".

Fix an instance where the article "the" before URL was missing.

* users (#6878, #3526, #4209): Validation error messages are now exposed through 400 responses, 409 response are sent in case of duplicate email or username
* MRs (#5335): 409 responses are sent in case of duplicate merge request (source/target branches), 422 responses are sent when submiting MR fo/from unrelated forks
* issues
* labels
* projects

Further changes

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

-Specifying a header of SUDO or adding a :sudo with either user id, or username of the user will set the current_user to be that user if your identifying private_token/PRIVATE_TOKEN is an administrator token

Updated the relative links in ## Contents to point to the correct documents.

 * Closing #3066
 * Closing #3609

All the info to return codes from the API functions are available in the `README.md` file as suggested.