doc: update documentation on '-k' parameter of utilities

doc/man_kdig.rst

@@ -63,11 +63,9 @@ Options
   Print help and usage.
 
 **-k** *keyfile*
-  Use TSIG or SIG-0 key stored in a file *keyfile* to authenticate the request.
-  Supported file format is the same as generated by ISC :program:`dnssec-keygen`.
-  The key comprises of public (.key extension) and private part (.private
-  extension). Either of these file names or a name without the extension can be
-  specified as *keyfile* parameter.
+  Use TSIG key stored in a file *keyfile* to authenticate the request. The
+  file must contain the key in the same format, which is accepted by the
+  **-y** option.
 
 **-p** *port*
   Set nameserver port number or service name to send a query to. The default

doc/man_knotc.rst

@@ -27,10 +27,12 @@ Parameters
   Remote server port (only for IP).
 
 **-y**, **--key** [*alg*:]\ *name*:*key*
-  Use key specified on the command line (default algorithm is hmac-md5).
+  Use TSIG key specified on the command line (default algorithm is hmac-md5).
 
 **-k**, **--keyfile** *file*
-  Use key file.
+  Use TSIG key stored in a file *file* to authenticate the request. The
+  file must contain the key in the same format, which is accepted by the
+  **-y** option.
 
 **-f**, **--force**
   Force operation. Overrides some checks.

doc/man_knsupdate.rst

@@ -29,11 +29,9 @@ Options
   Print help and usage.
 
 **-k** *keyfile*
-  Use TSIG or SIG-0 key stored in a file *keyfile* to authenticate the request.
-  Supported file format is the same as generated by ISC :program:`dnssec-keygen`.
-  The key comprises of public (.key extension) and private part (.private
-  extension). Either of these file names or a name without the extension can be
-  specified as *keyfile* parameter.
+  Use TSIG key stored in a file *keyfile* to authenticate the request. The
+  file should contain the key in the same format, which is accepted by the
+  **-y** option.
 
 **-p** *port*
   Set the port to use for connections to the server (if not explicitly specified