Improve the usage of variable names in C docstrings.

* libguile/alist.c:
* libguile/array-map.c:
* libguile/arrays.c:
* libguile/bitvectors.c:
* libguile/filesys.c:
* libguile/foreign.c:
* libguile/generalized-arrays.c:
* libguile/hashtab.c:
* libguile/ioext.c:
* libguile/load.c:
* libguile/numbers.c:
* libguile/ports.c:
* libguile/posix.c:
* libguile/print.c:
* libguile/procprop.c:
* libguile/promises.c:
* libguile/simpos.c:
* libguile/socket.c:
* libguile/srfi-1.c:
* libguile/srfi-13.c:
* libguile/srfi-14.c:
* libguile/stacks.c:
* libguile/stime.c:
* libguile/strings.c:
* libguile/struct.c:
* libguile/symbols.c:
* libguile/threads.c:
* libguile/weak-table.c:
* libguile/weak-vector.c: Make the variable names in the C docstrings more
  consistent.  Replace a few instances of @var with @code when appropriate.

libguile/load.c

@@ -571,9 +571,9 @@ SCM_DEFINE (scm_search_path, "search-path", 2, 0, 1,
 	    "@var{filename}. The file must be readable, and not a directory.\n"
 	    "If we find one, return its full filename; otherwise, return\n"
 	    "@code{#f}.  If @var{filename} is absolute, return it unchanged.\n"
-	    "If given, @var{extensions} is a list of strings; for each\n"
+	    "If given, @var{rest} is a list of extension strings; for each\n"
 	    "directory in @var{path}, we search for @var{filename}\n"
-	    "concatenated with each @var{extension}.")
+	    "concatenated with each extension.")
 #define FUNC_NAME s_scm_search_path
 {
   SCM extensions, require_exts;