docs(api): specify format for CI_PULL_REQUEST

docs/usage/configuration.rst

@@ -166,18 +166,25 @@ other CI systems can be supported if the following environment variables are
 defined::
 
     CI_NAME
-        # Name of the CI service being used
+        # Name of the CI service being used.
     CI_BUILD_NUMBER
         # The number assigned to the build by your CI service.
     CI_BUILD_URL
-        # URL to a webpage showing the build information/logs
+        # URL to a webpage showing the build information/logs.
     CI_BRANCH
         # For pull requests this is the name of the branch being targeted,
-        # otherwise it corresponds to the name of the current branch or tag
+        # otherwise it corresponds to the name of the current branch or tag.
     CI_JOB_ID (optional)
         # For parallel builds, the number assigned to each job comprising the build.
         # When missing, Coveralls will assign an incrementing integer (1, 2, 3 ...).
         # This value should not change between multiple runs of the build.
     CI_PULL_REQUEST (optional)
         # If given, corresponds to the number of the pull request, as specified
-        # in the supported repository hosting service (GitHub, GitLab, etc)
+        # in the supported repository hosting service (GitHub, GitLab, etc).
+        # This variable expects a value defined as an integer, e.g.:
+        #   CI_PULL_REQUEST=42             (recommended)
+        # However, for flexibility, any single line string ending with the same
+        # integer value can also be used (such as the pull request URL or
+        # relative path), e.g.:
+        #   CI_PULL_REQUEST='myuser/myrepo/pull/42'
+        #   CI_PULL_REQUEST='https://github.com/myuser/myrepo/pull/42'