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.

doc/manual.rst

@@ -3603,9 +3603,17 @@ does not provide a value for the argument. The value will be reevaluated
 every time the function is called.
 
 .. code-block:: nim
-  # b is optional with 47 as its default value
+  # b is optional with 47 as its default value.
   proc foo(a: int, b: int = 47): int
 
+Just as the comma propagates the types from right to left until the
+first parameter or until a semicolon is hit, it also propagates the
+default value starting from the parameter declared with it.
+
+.. code-block:: nim
+  # Both a and b are optional with 47 as their default values.
+  proc foo(a, b: int = 47): int
+
 Parameters can be declared mutable and so allow the proc to modify those
 arguments, by using the type modifier `var`.
 

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)