Help: Clean up how TLS and NETRC variables are discussed

Mention of the fallback of the command options to the CMAKE_...
variables was repeated in places. The wording was also a bit
unclear about whether the fallback only occurred when neither
TLS or neither NETRC option was given to the command. Move
the fallback details directly to each relevant option instead.

Also use cross-referencing to the variables where it was not
linked previously.

Help/command/file.rst

@@ -1073,7 +1073,7 @@ Options to both ``DOWNLOAD`` and ``UPLOAD`` are:
   .. versionadded:: 3.11
 
   Specify whether the .netrc file is to be used for operation.  If this
-  option is not specified, the value of the ``CMAKE_NETRC`` variable
+  option is not specified, the value of the :variable:`CMAKE_NETRC` variable
   will be used instead.
   Valid levels are:
 
@@ -1092,30 +1092,28 @@ Options to both ``DOWNLOAD`` and ``UPLOAD`` are:
 
   Specify an alternative .netrc file to the one in your home directory,
   if the ``NETRC`` level is ``OPTIONAL`` or ``REQUIRED``. If this option
-  is not specified, the value of the ``CMAKE_NETRC_FILE`` variable will
+  is not specified, the value of the :variable:`CMAKE_NETRC_FILE` variable will
   be used instead.
 
-If neither ``NETRC`` option is given CMake will check variables
-``CMAKE_NETRC`` and ``CMAKE_NETRC_FILE``, respectively.
-
 ``TLS_VERIFY <ON|OFF>``
   Specify whether to verify the server certificate for ``https://`` URLs.
-  The default is to *not* verify.
+  The default is to *not* verify. If this option is not specified, the value
+  of the :variable:`CMAKE_TLS_VERIFY` variable will be used instead.
 
   .. versionadded:: 3.18
     Added support to ``file(UPLOAD)``.
 
 ``TLS_CAINFO <file>``
-  Specify a custom Certificate Authority file for ``https://`` URLs.
+  Specify a custom Certificate Authority file for ``https://`` URLs. If this
+  option is not specified, the value of the :variable:`CMAKE_TLS_CAINFO`
+  variable will be used instead.
 
   .. versionadded:: 3.18
     Added support to ``file(UPLOAD)``.
 
 For ``https://`` URLs CMake must be built with OpenSSL support.  ``TLS/SSL``
 certificates are not checked by default.  Set ``TLS_VERIFY`` to ``ON`` to
-check certificates. If neither ``TLS`` option is given, CMake will use the
-settings from the :variable:`CMAKE_TLS_VERIFY` and :variable:`CMAKE_TLS_CAINFO`
-variables instead.
+check certificates.
 
 Additional options to ``DOWNLOAD`` are: