fix a few dead links and a missing sentence in documentation (#12387)

lib/pure/strscans.nim

@@ -76,7 +76,7 @@ One very nice advantage over regular expressions is that ``scanf`` is
 extensible with ordinary Nim procs. The proc is either enclosed in ``${}``
 or in ``$[]``. ``${}`` matches and binds the result
 to a variable (that was passed to the ``scanf`` macro) while ``$[]`` merely
-optional tokens.
+matches optional tokens without any result binding.
 
 
 In this example, we define a helper proc ``someSep`` that skips some separators

lib/system/io.nim

@@ -667,10 +667,10 @@ when defined(windows) and appType == "console" and
 
 proc readFile*(filename: string): TaintedString {.tags: [ReadIOEffect], benign.} =
   ## Opens a file named `filename` for reading, calls `readAll
-  ## <#readAll>`_ and closes the file afterwards. Returns the string.
-  ## Raises an IO exception in case of an error. If # you need to call
+  ## <#readAll,File>`_ and closes the file afterwards. Returns the string.
+  ## Raises an IO exception in case of an error. If you need to call
   ## this inside a compile time macro you can use `staticRead
-  ## <#staticRead>`_.
+  ## <system.html#staticRead,string>`_.
   var f: File
   if open(f, filename):
     try: