[PATCH] decodetree.rst: add hint about format reuse with '.'

[Daniel Henrique Barboza <danielhb413@gmail.com>]

This observation came up during the review of "Move tlbie[l] to decode
tree" patch [1] and it seems useful enough to be added in the docs.

[1] https://lists.gnu.org/archive/html/qemu-devel/2022-07/msg02195.html

Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
---
 docs/devel/decodetree.rst | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/docs/devel/decodetree.rst b/docs/devel/decodetree.rst
index 49ea50c2a7..6bcb94f449 100644
--- a/docs/devel/decodetree.rst
+++ b/docs/devel/decodetree.rst
@@ -110,7 +110,9 @@ A *fixedbit_elt* describes a contiguous sequence of bits that must
 be 1, 0, or don't care.  The difference between '.' and '-'
 is that '.' means that the bit will be covered with a field or a
 final 0 or 1 from the pattern, and '-' means that the bit is really
-ignored by the cpu and will not be specified.
+ignored by the cpu and will not be specified.  The same format can
+be reused in multiple patterns by using '.' and let the pattern
+decide whether the given bit will be ignored or not.
 
 A *field_elt* describes a simple field only given a width; the position of
 the field is implied by its position with respect to other *fixedbit_elt*
-- 
2.36.1