docs: Add linter signatures and terraform example

Author: mothzarella

doc/guard.nvim.txt

@@ -637,6 +637,42 @@ That’s what the attributes field does, it extracts them from the json table:
 Et voilà!
 
 
+SIGNATURES FOR CUSTOM LINTER FUNCTIONS          *guard.nvim-linter-signatures*
+
+When writing custom lint logic, `fn` and `parse` receive buffer context:
+
+>lua
+    lint.fn(prev_lines, fname, cwd)
+    lint.parse(result, bufnr, fname, cwd)
+<
+
+Parameters ~
+  • `prev_lines` — buffer content (string)
+  • `result`     — linter output (string)  
+  • `bufnr`      — target buffer number
+  • `fname`      — absolute file path
+  • `cwd`        — working directory
+
+Use `fname` and `cwd` when linters output relative paths (e.g., 
+`terraform validate` on a directory returns diagnostics for all files).
+
+Filtering `terraform validate` by current file:
+
+>lua
+    parse = function(result, bufnr, fname, cwd)
+	local current = fname:sub(#cwd + 2) -- +2 to include '/'
+        local decoded = vim.json.decode(result)
+        
+        for _, d in ipairs(decoded.diagnostics or {}) do
+            -- terraform returns relative filenames; match against current
+            if d.range and d.range.filename == current then
+                -- add to diagnostics...
+            end
+        end
+    end
+<
+
+
 TAKE ADVANTAGE OF AUTOCMD EVENTS*guard.nvim-advanced-tips-take-advantage-of-autocmd-events*
 
 Guard exposes a `GuardFmt` user event that you can use. It is called both