rc man/doc: Don't show groff warnings

Sometimes the implementation of `man` will display errors, e.g.

```
<standard input>:4808: warning [p 54, 13.2i]: can't break line
```

Those errors are harmless but are still reported on the debug buffer,
so we hide them by redirecting the standard error stream to /dev/null.

rc/core/doc.kak

@@ -8,7 +8,7 @@ def -hidden -params 1..2 doc-open %{
         # This option is handled by the `man-db` implementation
         export MANWIDTH=${kak_window_width}
 
-        if man "$1" > "${manout}"; then
+        if man "$1" > "${manout}" 2>/dev/null; then
             readonly manout_noescape=$(mktemp "${TMPDIR:-/tmp}"/kak-man-XXXXXX)
 
             sed -e $(printf 's/.\x8//g') -e 's,\x1B\[[0-9;]*[a-zA-Z],,g' "${manout}" > "${manout_noescape}"

rc/core/man.kak

@@ -30,7 +30,7 @@ hook global WinSetOption filetype=(?!man).* %{
 def -hidden -params 1..2 man-impl %{ %sh{
     manout=$(mktemp "${TMPDIR:-/tmp}"/kak-man-XXXXXX)
     colout=$(mktemp "${TMPDIR:-/tmp}"/kak-man-XXXXXX)
-    MANWIDTH=${kak_window_width} man "$@" > $manout
+    MANWIDTH=${kak_window_width} man "$@" > $manout 2>/dev/null
     retval=$?
     col -b -x > ${colout} < ${manout}
     rm ${manout}