spec: clarify that signed integers>=0 are permitted as shift counts

In Go1.13 and above, signed integers are permitted as shift counts as long as they are >=0.
However, the comments in the "Arithmetic operators" section says shift operators accept "unsigned integer" as of right operands. Replacing this with "integer>=0" resolves the misunderstanding that shift
operators permit only unsigned integers.

Reference: Go1.13 Release Notes: https://golang.org/doc/go1.13

Change-Id: Icd3c7734d539ab702590e992a618c9251c653c37
GitHub-Last-Rev: 4f263a4
GitHub-Pull-Request: #44664
Reviewed-on: https://go-review.googlesource.com/c/go/+/297249
Reviewed-by: Emmanuel Odeke <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
Trust: Emmanuel Odeke <[email protected]>
Trust: Robert Griesemer <[email protected]>

Author: nobishino

doc/go_spec.html

@@ -1,6 +1,6 @@
 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of Feb 24, 2021",
+	"Subtitle": "Version of Mar 16, 2021",
 	"Path": "/ref/spec"
 }-->
 
@@ -3681,8 +3681,8 @@ <h3 id="Arithmetic_operators">Arithmetic operators</h3>
 ^    bitwise XOR            integers
 &amp;^   bit clear (AND NOT)    integers
 
-&lt;&lt;   left shift             integer &lt;&lt; unsigned integer
-&gt;&gt;   right shift            integer &gt;&gt; unsigned integer
+&lt;&lt;   left shift             integer &lt;&lt; integer >= 0
+&gt;&gt;   right shift            integer &gt;&gt; integer >= 0
 </pre>