Update :Validate command definition to prevent error

This change updates the :Validate command definition to drop the
-complete=file option, and updates the docs to indicate the :Validate
command does not take a file argument.

Otherwise, without this change, loading of eclim/ftplugin/xml.vim in
versions of vim later than 8.2.3140 will fail with the error message
“E1208: -complete used without -nargs” — due to this vim change:

vim/vim@de69a73

That change causes vim to explicitly check for the case where -nargs=0
is used with -complete — because it doesn’t seem to make sense to use
-complete without also actually permitting arguments for completion.

And in org.eclim.core/vim/eclim/autoload/eclim/xml/validate.vim, there
doesn’t actually seem to be any support for handling any file argument
passed to the :Validate command. So that’s why this patch drops the
-complete=file option — rather than setting -nargs=1 — and that’s why
the patch also updates the docs to indicate :Validate takes no argument.

Fixes #615

Related: vim/vim#8544

doc/content/vim/xml/index.rst

@@ -99,8 +99,8 @@ following command is still available for validating xml files on demand.
 
 .. _\:Validate_xml:
 
-**:Validate** [<file>] -
-Validate the supplied file or the current file if no file name provided.
+**:Validate** -
+Validate the the current file.
 
 If eclimd is not currently running, and the xmllint command is available,
 eclim will validate the xml file using that.  Eclim will never use xmllint

org.eclim.core/vim/eclim/ftplugin/xml.vim

@@ -33,7 +33,7 @@ endif
 " Command Declarations {{{
 
 if !exists(":Validate")
-  command -nargs=0 -complete=file -bang -buffer Validate
+  command -nargs=0 -bang -buffer Validate
     \ :call eclim#xml#validate#Validate(0, '<bang>')
 
   command -nargs=? -buffer DtdDefinition