doc: small UX improvements.

Place the cursor at the beginning of the file (and not at
the end of the first line) and improve error message in
case of failure.

rc/tools/doc.kak

@@ -125,6 +125,8 @@ define-command -params 1 -hidden doc-render %{
 
     # Remove escaping of * and `
     try %{ execute-keys -draft <percent> s \\((?=\*)|(?=`)) <ret> d }
+    # Go to beginning of file
+    execute-keys 'gg'
 
     set-option buffer readonly true
     add-highlighter buffer/ ranges doc_render_ranges
@@ -184,7 +186,7 @@ define-command -params 0..2 \
             fi
             printf %s\\n "evaluate-commands -try-client %opt{docsclient} %{ doc-render ${page}; ${jump_cmd} }"
         else
-            printf 'fail No such doc file: %s\n' "${page}"
+            printf 'fail No such doc file: %s\n' "$topic.asciidoc"
         fi
     }
 }