Add doxygen block for environment_create_named

Andrew flagged the new helper introduced for the value-pool-leak fix
as missing documentation. akbasic_environment_create() and
akbasic_environment_create_empty() are already documented in the
header; the shared static helper they both call was the only
undocumented new function definition.

Co-authored-by: andrew <andrew@aklabs.net>

src/environment.c

@@ -306,6 +306,21 @@ akerr_ErrorContext *akbasic_environment_get(akbasic_Environment *obj, const char
     SUCCEED_RETURN(errctx);
 }
 
+/**
+ * @brief Create a variable slot in the given scope, optionally allocating its storage.
+ *
+ * Shared by akbasic_environment_create() and akbasic_environment_create_empty(),
+ * which differ only in whether the new variable's value storage is initialized
+ * immediately or left for the caller to set up.
+ *
+ * @param obj Scope the variable is created in; only this scope is searched or
+ *        written to, unlike akbasic_environment_get()'s walk up the parent chain.
+ * @param varname Name of the variable to create.
+ * @param dest Set to the created (or already-existing) variable.
+ * @param initialize When true, the variable's value storage is allocated from
+ *        the runtime's value pool; when false, the caller must initialize it
+ *        before the variable is evaluated.
+ */
 static akerr_ErrorContext *environment_create_named(akbasic_Environment *obj, const char *varname,
 							akbasic_Variable **dest, bool initialize)
 {