ci: Document what makes certain tasks special

To increase coverage without drastically increasing CI resource usage, we have
different CI tasks test different things (e.g. the linux tasks use
sanitizers).  Unfortunately that can create confusing situations where CI
fails on some OS, but not others, without the problem appearing to be platform
dependent.

To, partially, address that, add a comment, prefixed with SPECIAL, to each
task that we use to test in some non-default way.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/321570.1741195755@sss.pgh.pa.us

.cirrus.tasks.yml

@@ -2,6 +2,12 @@
 #
 # For instructions on how to enable the CI integration in a repository and
 # further details, see src/tools/ci/README
+#
+#
+# NB: Different tasks intentionally test with different, non-default,
+# configurations, to increase the chance of catching problems. Each task with
+# non-obvious non-default documents their oddity at the top of the task,
+# prefixed by "SPECIAL:".
 
 
 env:
@@ -55,6 +61,10 @@ on_failure_meson: &on_failure_meson
 
 # To avoid unnecessarily spinning up a lot of VMs / containers for entirely
 # broken commits, have a minimal task that all others depend on.
+#
+# SPECIAL:
+# - Builds with --auto-features=disabled and thus almost no enabled
+#   dependencies
 task:
   name: SanityCheck
 
@@ -125,6 +135,11 @@ task:
       src/tools/ci/cores_backtrace.sh linux /tmp/cores
 
 
+# SPECIAL:
+# - Uses postgres specific CPPFLAGS that increase test coverage
+# - Specifies configuration options that test reading/writing/copying of node trees
+# - Specifies debug_parallel_query=regress, to catch related issues during CI
+# - Also runs tests against a running postgres instance, see test_running_script
 task:
   name: FreeBSD - Meson
 
@@ -355,6 +370,7 @@ LINUX_MESON_FEATURES: &LINUX_MESON_FEATURES >-
   -Duuid=e2fs
 
 
+# Check SPECIAL in the matrix: below
 task:
   env:
     CPUS: 4
@@ -435,6 +451,10 @@ task:
       libcurl4-openssl-dev:i386 \
 
   matrix:
+    # SPECIAL:
+    # - Uses address sanitizer, sanitizer failures are typically printed in
+    #   the server log
+    # - Configures postgres with a small segment size
     - name: Linux - Debian Bookworm - Autoconf
 
       env:
@@ -470,6 +490,10 @@ task:
       on_failure:
         <<: *on_failure_ac
 
+    # SPECIAL:
+    # - Uses undefined behaviour and alignment sanitizers, sanitizer failures
+    #   are typically printed in the server log
+    # - Test both 64bit and 32 bit builds
     - name: Linux - Debian Bookworm - Meson
 
       env:
@@ -530,6 +554,11 @@ task:
     cores_script: src/tools/ci/cores_backtrace.sh linux /tmp/cores
 
 
+# NB: macOS is by far the most expensive OS to run CI for, therefore no
+# expensive additional checks should be added.
+#
+# SPECIAL:
+# - Enables --clone for pg_upgrade and pg_combinebackup
 task:
   name: macOS - Sonoma - Meson