Document references of libmongoc build tasks

Author: alcaeus

.evergreen/config/build-task-groups.yml

@@ -32,7 +32,7 @@ task_groups:
       - ".build !.php7.4 !.php8.0"
 
   - name: "build-php-libmongoc"
-    # Keep this in sync with the actual number of libmongoc builds (typically 3)
+    # Keep this in sync with the actual number of libmongoc builds (typically 3) defined in _template-build-libmongoc.yml
     max_hosts: 1
     setup_task: *build_setup
     setup_task_can_fail_task: true

.evergreen/config/build/_template-build-libmongoc.yml

@@ -1,3 +1,9 @@
+# The following tasks are used to build with different libmongoc versions.
+# If libmongoc is not set to a stable version, disable the first two builds and only leave the last one enabled.
+# When updating libmongoc to a stable release, make sure all builds are enabled.
+# These tasks are used in the build task group "build-php-libmongoc" defined in .evergreen/config/build-task-groups.yml,
+# where the "max_hosts" setting must be set to the number of enabled tasks.
+# Last but not least, make sure the corresponding test variants are enabled in .evergreen/config/test-variants.yml
 tasks:
   # TODO: Enable the following two builds once libmongoc 1.25.0 is released
 #  - name: "build-php-%phpVersion%-libmongoc-lowest-supported"