Api: Adding cterm color support to nvim_set_hl

src/nvim/api/vim.c

@@ -217,6 +217,12 @@ Dictionary nvim__get_hl_defs(Integer ns_id, Error *err)
 ///            in addition the following keys are also recognized:
 ///              `default`: don't override existing definition,
 ///                         like `hi default`
+///              `ctermfg`: sets foreground of cterm color
+///              `ctermbg`: sets background of cterm color
+///              `cterm`  : cterm attribute map. sets attributed for
+///                         cterm colors. similer to `hi cterm`
+///                         Note: by default cterm attributes are 
+///                               same as attributes of gui color
 /// @param[out] err Error details, if any
 ///
 /// TODO: ns_id = 0, should modify :highlight namespace