(syms_of_coding): Doc fix.

src/coding.c

@@ -5673,11 +5673,16 @@ There are three such tables, `file-coding-system-alist',\n\
 
   DEFVAR_LISP ("coding-system-for-write", &Vcoding_system_for_write,
     "Specify the coding system for write operations.\n\
-It is useful to bind this variable with `let', but do not set it globally.\n\
-If the value is a coding system, it is used for encoding on write operation.\n\
-If not, an appropriate element is used from one of the coding system alists:\n\
+Programs bind this variable with `let', but you should not set it globally.\n\
+If the value is a coding system, it is used for encoding of output,\n\
+when writing it to a file and when sending it to a file or subprocess.\n\
+\n\
+If this does not specify a coding system, an appropriate element\n\
+is used from one of the coding system alists:\n\
 There are three such tables, `file-coding-system-alist',\n\
-`process-coding-system-alist', and `network-coding-system-alist'.");
+`process-coding-system-alist', and `network-coding-system-alist'.\n\
+For output to files, if the above procedure does not specify a coding system,\n\
+the value of `buffer-file-coding-system' is used.");
   Vcoding_system_for_write = Qnil;
 
   DEFVAR_LISP ("last-coding-system-used", &Vlast_coding_system_used,