Add docstrings for value(::Missing) & uncertainty(::Missing)

Author: Eben60

src/utils.jl

@@ -71,6 +71,7 @@ uncertainty(a::Missing) = missing
 """
     Measurements.value(x::Measurement)
     Measurements.value(x::Complex{Measurement})
+    Measurements.value(x::Missing)
 
 Return the nominal value of measurement `x`.
 """
@@ -79,6 +80,7 @@ value
 """
     Measurements.uncertainty(x::Measurement)
     Measurements.uncertainty(x::Complex{Measurement})
+    Measurements.uncertainty(x::Missing)
 
 Return the uncertainty of measurement `x`.
 """