fixes #4579

doc/manual/ffi.txt

@@ -16,12 +16,20 @@ spelled*:
 .. code-block::
   proc printf(formatstr: cstring) {.header: "<stdio.h>", importc: "printf", varargs.}
 
-Note that this pragma is somewhat of a misnomer: Other backends will provide
+Note that this pragma is somewhat of a misnomer: Other backends do provide
 the same feature under the same name. Also, if one is interfacing with C++
 the `ImportCpp pragma <manual.html#implementation-specific-pragmas-importcpp-pragma>`_ and
 interfacing with Objective-C the `ImportObjC pragma
 <manual.html#implementation-specific-pragmas-importobjc-pragma>`_ can be used.
 
+The string literal passed to ``importc`` can be a format string:
+
+.. code-block:: Nim
+  proc p(s: cstring) {.importc: "prefix$1".}
+
+In the example the external name of ``p`` is set to ``prefixp``. Only ``$1``
+is available and a literal dollar sign must be written as ``$$``.
+
 
 Exportc pragma
 --------------
@@ -33,9 +41,19 @@ name is the Nim identifier *exactly as spelled*:
 .. code-block:: Nim
   proc callme(formatstr: cstring) {.exportc: "callMe", varargs.}
 
-Note that this pragma is somewhat of a misnomer: Other backends will provide
+Note that this pragma is somewhat of a misnomer: Other backends do provide
 the same feature under the same name.
 
+The string literal passed to ``exportc`` can be a format string:
+
+.. code-block:: Nim
+  proc p(s: string) {.exportc: "prefix$1".} =
+    echo s
+
+In the example the external name of ``p`` is set to ``prefixp``. Only ``$1``
+is available and a literal dollar sign must be written as ``$$``.
+
+
 
 Extern pragma
 -------------
@@ -46,7 +64,9 @@ mangling. The string literal passed to ``extern`` can be a format string:
   proc p(s: string) {.extern: "prefix$1".} =
     echo s
 
-In the example the external name of ``p`` is set to ``prefixp``.
+In the example the external name of ``p`` is set to ``prefixp``. Only ``$1``
+is available and a literal dollar sign must be written as ``$$``.
+
 
 
 Bycopy pragma