website: add docs for variable warning bypass opts

When the options for bypassing/enabling assigned and undeclared
variables were added to Packer, the website documentation for those
commands and options was not updated.

This commit adds some documentation for those options.

website/content/docs/commands/build.mdx

@@ -58,3 +58,11 @@ artifacts that are created will be outputted at the end of the build.
   multiple times. This is useful for setting version numbers for your build.
 
 - `-var-file` - Set template variables from a file.
+
+- `-warn-on-undeclared-var` - Setting this flag will yield a warning for each assignment within
+  a variable definitions file (*.pkrvars.hcl | *.pkrvars.json) that does not have an accompanying
+  variable block. This can occur when using a var-file that contains a large amount of unused variables
+  for a given HCL2 template. For HCL2 template builds defining a value for a variable in a var-file is
+  not enough on its own for Packer to function, as there also needs to be a variable block definition in
+  the template files `pkr.hcl` for the variable. By default `packer build` will not warn when a var-file
+  contains one or more undeclared variables.

website/content/docs/commands/validate.mdx

@@ -47,6 +47,15 @@ Errors validating build 'vmware'. 1 error(s) occurred:
   source block's "name" label, unless an in-build source definition adds the
   "name" configuration option.
 
+- `-no-warn-on-undeclared-var` - Silence warnings when the a variable definition
+  file contains variable assignments for undeclared variables. This can occur
+  when using a var-file that contains a large amount of unused variables for a
+  given HCL2 template. For HCL2 template defining a value for a variable in a
+  var-file is not enough on its own for Packer to function, as there also needs
+  to be a variable block definition in the template files `pkr.hcl` for the
+  variable. By default `packer validate` will warn when a var-file contains one
+  or more undeclared variables.
+
 - `-only=foo,bar,baz` - Only validate the builds with the given comma-separated
   names. In legacy JSON templates, build names default to the
   types of their builders (e.g. `docker` or