clarify library versioning and Go version compatibility in document

docs/api.md

@@ -35,10 +35,21 @@ Followings are unexhaustive list of interesting APIs.
   found by the validator.
 - `ActionMetadata` is a struct for action metadata file (`action.yml`). It is used to check inputs specified at `with:`
   and typing `steps.{id}.outputs` object strictly.
-- `PopularActions` global variable is data set of popular actions' metadata collected by [the script](../scripts/generate-popular-actions).
+- `PopularActions` global variable is the data set of popular actions' metadata collected by [the script](../scripts/generate-popular-actions).
+- `AllWebhookTypes` global variable is the mapping from all webhook names to their types collected by [the scirpt](../scripts/generate-webhook-events).
+- `WorkflowKeyAvailability()` returns available context names and special function names for the given workflow key like
+  `jobs.<job_id>.outputs.<output_id>`. This function uses the data collected by [the scirpt](../scripts/generate-availability).
 
-Note that the version of this repository is for command line tool `actionlint`. So it does not represent version of the
-library. It means that patch version bump may introduce some breaking changes.
+## Library versioning
+
+The version of this repository is for command line tool `actionlint`. So it does not represent the version of the library.
+It means that the library does not follow sematinc versioning and any patch version bump may introduce some breaking changes.
+
+## Go version compatibility
+
+Minimum supported Go version is written in [`go.mod`](../go.mod) file in this repository. That said, older Go versions are
+actually not tested on CI. Last two major Go versions are recommended because they're tested on CI. For example, when the latest
+Go version is v1.22, v1.21 and v1.22 are nice to use.
 
 ---
 

error.go

@@ -252,7 +252,6 @@ func (by byRuleNameField) Swap(i, j int) {
 // ErrorFormatter is a formatter to format a slice of ErrorTemplateFields. It is used for
 // formatting error messages with -format option.
 type ErrorFormatter struct {
-	// Note: Evaluating a template is thread-safe.  // > // https://pkg.go.dev/text/template
 	temp    *template.Template
 	rules   map[string]*ruleTemplateFields
 	rulesMu sync.Mutex