docstring for @time_imports: explain what is shown (it's not cumulative)

Re
- JuliaLang#45861 (comment)

Author: tfiers

stdlib/InteractiveUtils/src/macros.jl

@@ -360,6 +360,9 @@ See also: [`code_native`](@ref), [`@code_llvm`](@ref), [`@code_typed`](@ref) and
 A macro to execute an expression and produce a report of any time spent importing packages and their
 dependencies. Any compilation time will be reported as a percentage, and how much of which was recompilation, if any.
 
+One line is printed per package. The duration shown is the time to import that package itself
+(and not any of its dependencies).
+
 On Julia 1.9+ [package extensions](@ref man-extensions) will show as Parent → Extension.
 
 !!! note