docs/manual: document new behaviour of 'make printvars'

Add a 'VARS=...' setting to the example. To make it clear that several
variables can be specified, use two variables in the first example.
Only 2 variables are printed, so the ... is removed.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

docs/manual/make-tips.txt

@@ -80,18 +80,18 @@ to xref:ccache[].
 
 .Dumping the internal make variables:
 
-One can dump all the variables known to make, along with their values:
+One can dump the variables known to make, along with their values:
 
 ----
- $ make -s printvars
- VARIABLE=value_of_variable
- ...
+ $ make -s printvars VARS='VARIABLE1 VARIABLE2'
+ VARIABLE1=value_of_variable
+ VARIABLE2=value_of_variable
 ----
 
 It is possible to tweak the output using some variables:
 
 - +VARS+ will limit the listing to variables which names match the
-  specified make-pattern
+  specified make-patterns - this must be set else nothing is printed
 - +QUOTED_VARS+, if set to +YES+, will single-quote the value
 - +RAW_VARS+, if set to +YES+, will print the unexpanded value