Align comments in .dockerignore with synapse-private pro branch (#20224)

See https://github.com/element-hq/synapse-private/blob/pro/.dockerignore

Spawning from https://github.com/element-hq/synapse-private/issues/169
where because we now have 'Synapse Variants' builds for the Synapse Pro
container images, we can revert our special `docker/` changes back to
match this repo. In this case, the new comments here make some sense so
let's keep it ⏩

.dockerignore

@@ -1,7 +1,7 @@
 # ignore everything by default
 *
 
-# things to include
+# Things to include
 !docker
 !synapse
 !rust
@@ -12,6 +12,7 @@
 !Cargo.toml
 !build_rust.py
 
+# Things to re-ignore
 rust/target
 synapse/*.so
 

changelog.d/20224.misc

@@ -0,0 +1 @@
+Align comments in `.dockerignore` with what we have in the private Synapse Pro branch.