Add precision in function help that output_file is expected to be a…

… filename

`quarto render --output` will throw the error if the value provided is not supported. We don't want to add its own validation system in the package. If this is not clear enough, we could try to catch the error and rethrow differently, but this would still mean we need to maintain two error system for same usage.

Closes #89

R/render.R

@@ -8,8 +8,10 @@
 #'   to rendering the project in the current working directory).
 #' @param output_format Target output format (defaults to "html"). The option
 #'   `"all"` will render all formats defined within the file or project.
-#' @param output_file The name of the output file. If using `NULL` then the
-#'   output filename will be based on filename for the input file.
+#' @param output_file The name of the output file. If using `NULL`, the output
+#'   filename will be based on the filename for the input file. `output_file` is
+#'   mapped to the `--output` option flag of the `quarto` CLI. It is expected to
+#'   be a filename only, not a path, relative or absolute.
 #' @param execute Whether to execute embedded code chunks.
 #' @param execute_params A list of named parameters that override custom params
 #'   specified within the YAML front-matter.