Clarify mathematical definition of gcd (#57204)

See also:

Triage request for this:
#57067 (comment)
Similar PR for `lcm`: #56992

(cherry picked from commit 41a4dfa)

Author: LilithHafner

base/intfuncs.jl

@@ -8,6 +8,9 @@
 Greatest common (positive) divisor (or zero if all arguments are zero).
 The arguments may be integer and rational numbers.
 
+``a`` is a divisor of ``b`` if there exists an integer ``m`` such
+that ``ma=b``.
+
 !!! compat "Julia 1.4"
     Rational arguments require Julia 1.4 or later.