Audit/fix documentation in doc/api/runners.md

The documentation of the endpoints in doc/api/runners.md is not 100% correct and needs to be audited. As an example:

• some endpoints mention that you need to be a project owner in order to list available runners, but the implementation is only requiring being a maintainer.
• the GET /runners/:id endpoint mentions that You must have at least the Developer role for the target namespace or project., however the implementation is returning group runners where the user is owner of the associated group, and project runners where the user is a maintainer of any of the associated projects.

Draft patch

diff --git a/doc/api/runners.md b/doc/api/runners.md
index 5c1550bf4901..c7e55b7ac7f2 100644
--- a/doc/api/runners.md
+++ b/doc/api/runners.md
@@ -51,9 +51,8 @@ Get a list of runners available to the user.
 
 Prerequisites:
 
-- For instance runners, you must be an administrator of the GitLab instance.
 - For group runners, you must have the Owner role in the owner namespace.
-- For project runners, you must have at least the Maintainer role in the owner project.
+- For project runners, you must have at least the Maintainer role in a project assigned to the runner.
 
@@ -267,7 +266,7 @@ Instance runner details are available to all authenticated users through this en
 
 Prerequisites:
 
-- You must have at least the Developer role for the namespace or project associated with the runner.
+- You must have at least the Maintainer role for the namespace or project associated with the runner.
 - An access token with the `manage_runner` scope and the appropriate role.
 
@@ -363,7 +362,7 @@ Prerequisites:
 
 - For instance runners, you must be an administrator of the GitLab instance.
 - For group runners, you must have the Owner role in the owner namespace.
-- For project runners, you must have at least the Maintainer role in the owner project.
+- For project runners, you must have at least the Maintainer role in a project assigned to the runner.
 - An access token with the `manage_runner` scope and the appropriate role.
 
 | Attribute          | Type    | Required | Description |
@@ -451,7 +450,7 @@ Prerequisites:
 
 - For instance runners, you must be an administrator of the GitLab instance.
 - For group runners, you must have the Owner role in the owner namespace.
-- For project runners, you must have at least the Maintainer role in the owner project.
+- For project runners, you must have at least the Maintainer role in a project assigned to the runner.
 - An access token with the `manage_runner` scope and the appropriate role.
 
@@ -958,7 +957,7 @@ Prerequisites:
 
 - For instance runners, you must be an administrator of the GitLab instance.
 - For group runners, you must have the Owner role in the owner namespace.
-- For project runners, you must have at least the Maintainer role in the owner project.
+- For project runners, you must have at least the Maintainer role in a project assigned to the runner.
 - An access token with the `manage_runner` scope and the appropriate role.
 
@@ -981,7 +980,7 @@ Prerequisites:
 
 - For instance runners, you must be an administrator of the GitLab instance.
 - For group runners, you must have the Owner role in the owner namespace.
-- For project runners, you must have at least the Maintainer role in the owner project.
+- For project runners, you must have at least the Maintainer role in a project assigned to the runner.
 
@@ -1121,7 +1120,7 @@ Prerequisites:
 
 - For instance runners, you must be an administrator of the GitLab instance.
 - For group runners, you must have the Owner role in the owner namespace.
-- For project runners, you must have at least the Maintainer role in the owner project.
+- For project runners, you must have at least the Maintainer role in a project assigned to the runner.
 - An access token with the `manage_runner` scope and the appropriate role.