Update doc for multinomial

Author: inkydragon

src/factorials.jl

@@ -95,7 +95,34 @@ end
 """
     multinomial(k...)
 
-Multinomial coefficient where `n = sum(k)`.
+Compute the multinomial coefficient
+``\\binom{n}{k_1,k_2,...,k_i} = \\frac{n!}{k_1!k_2! \\cdots k_i!}, n = \\sum{k_i}``.
+Throws an `OverflowError` when the input is too large.
+
+See Also: `binomial`.
+
+# Examples
+```jldoctest
+julia> # (x+y)^2 = x^2 + 2xy + y^2
+
+julia> multinomial(2, 0)
+1
+
+julia> multinomial(1, 1)
+2
+
+julia> multinomial(0, 2)
+1
+
+julia> multinomial(10, 10, 10, 10)
+ERROR: OverflowError: 5550996791340 * 847660528 overflowed for type Int64
+Stacktrace:
+[...]
+```
+
+# External links
+- [Definitions](https://dlmf.nist.gov/26.4.2) on DLMF
+- [Multinomial theorem](https://en.wikipedia.org/wiki/Multinomial_theorem) on Wikipedia
 """
 function multinomial(k...)
     s = 0