[Patch v3] docs: Correct docs for extra_guest_irqs command line option

[Andrew Cooper <andrew.cooper3@citrix.com>]

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
CC: Ian Campbell <Ian.Campbell@citrix.com>
CC: Ian Jackson <Ian.Jackson@eu.citrix.com>

---
Changes for v3:
 * Further clarification
Changes for v2:
 * Spelling
 * Clarification about the effects of optional parameters
---
 docs/misc/xen-command-line.markdown |   10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/docs/misc/xen-command-line.markdown b/docs/misc/xen-command-line.markdown
index 708ffc2..202d800 100644
--- a/docs/misc/xen-command-line.markdown
+++ b/docs/misc/xen-command-line.markdown
@@ -485,9 +485,15 @@ disable it (edid=no). This option should not normally be required
 except for debugging purposes.
 
 ### extra\_guest\_irqs
-> `= <number>`
+> `= [<domU number>][,<dom0 number>]`
 
-Increase the number of PIRQs available for the guest. The default is 32. 
+> Default: `32,256`
+
+Change the number of PIRQs available for guests.  The optional first number is
+common for all domUs, while the optional second number (preceded by a comma)
+is for dom0.  Changing the setting for domU has no impact on dom0 and vice
+versa.  For example to change dom0 without changing domU, use
+`extra_guest_irqs=,512`
 
 ### flask\_enabled
 > `= <integer>`
-- 
1.7.10.4