docs: "How Terraform Uses Unicode" should mention HCL too

I missed this on my first attempt to write this document. Consequently
we're currently depending on a version of HCL which uses Unicode 9, and
that's significantly lagging behind everything else which is currently on
Unicode 13.

My goal of adding these docs then is to remind us to update HCL to Unicode
15 once we're updating everything else to Unicode 15 with the Go 1.20
release, assuming that the Go team completes that Unicode upgrade as
currently planned.

docs/unicode.md

@@ -45,6 +45,31 @@ The other subsystems described below should always be set up to match
 themselves with `unicode.Version` and generate an error if they cannot, but
 that isn't true of all of them.
 
+## Unicode Identifier Rules in HCL
+
+_Identifier and Pattern Syntax_ (TF31) is a Unicode standards annex which
+describe a set of rules for tokenizing "identifiers", such as variable names
+in a programming language.
+
+HCL uses a superset of that specification for its own identifier tokenization
+rules, and so it includes some code derived from the TF31 data tables that
+describe which characters belong to the "ID_Start" and "ID_Continue" classes.
+
+Since Terraform is the primary user of HCL, it's typically Terraform's adoption
+of a new Unicode version which drives HCL to adopt one. To update the Unicode
+tables to a new version:
+* Edit `hclsyntax/generate.go`'s line which runs `unicode2ragel.rb` to specify
+  the URL of the `DerivedCoreProperties.txt` data file for the intended Unicode
+  version.
+* Run `go generate ./hclsyntax` to run the generation code to update both
+  `unicode_derived.rl` and, indirectly, `scan_tokens.go`. (You will need both
+  a Ruby interpreter and the Ragel state machine compiler on your system in
+  order to complete this step.)
+* Run all the tests to check for regressions: `go test ./...`
+* If all looks good, commit all of the changes and open a PR to HCL.
+* Once that PR is merged and released, update Terraform to use the new version
+  of HCL.
+
 ## Unicode Text Segmentation
 
 _Text Segmentation_ (TR29) is a Unicode standards annex which describes