manual: Document that comma propagates the default values of parameters (#19080)

* manual: Document that comma propagates the default values of parameters

Fixes https://github.com/nim-lang/Nim/issues/15949.

* Use the "bug #NNNN" comment syntax for consistency

Ref:
https://nim-lang.github.io/Nim/contributing.html#writing-tests-stdlib

> Always refer to a GitHub issue using the following exact syntax: bug
for tooling.

tests/proc/t15949.nim

@@ -0,0 +1,20 @@
+# bug #15949
+
+discard """
+errormsg: "parameter 'a' requires a type"
+nimout: '''
+t15949.nim(20, 14) Error: parameter 'a' requires a type'''
+"""
+
+
+# line 10
+proc procGood(a, b = 1): (int, int) = (a, b)
+
+doAssert procGood() == (1, 1)
+doAssert procGood(b = 3) == (1, 3)
+doAssert procGood(a = 2) == (2, 1)
+doAssert procGood(a = 5, b = 6) == (5, 6)
+
+# The type (and default value propagation breaks in the below example
+# as semicolon is used instead of comma.
+proc procBad(a; b = 1): (int, int) = (a, b)