add explanation about top-level compilation to timing macros [skip ci] (#39802)

Author: IanButterworth

base/timing.jl

@@ -172,6 +172,10 @@ allocations, and the total number of bytes its execution caused to be allocated,
 returning the value of the expression. Any time spent garbage collecting (gc) or
 compiling is shown as a percentage.
 
+In some cases the system will look inside the `@time` expression and compile some of the
+called code before execution of the top-level expression begins. When that happens, some
+compilation time will not be counted. To include this time you can run `@time @eval ...`.
+
 See also [`@timev`](@ref), [`@timed`](@ref), [`@elapsed`](@ref), and
 [`@allocated`](@ref).
 
@@ -264,6 +268,10 @@ end
 A macro to evaluate an expression, discarding the resulting value, instead returning the
 number of seconds it took to execute as a floating-point number.
 
+In some cases the system will look inside the `@elapsed` expression and compile some of the
+called code before execution of the top-level expression begins. When that happens, some
+compilation time will not be counted. To include this time you can run `@elapsed @eval ...`.
+
 See also [`@time`](@ref), [`@timev`](@ref), [`@timed`](@ref),
 and [`@allocated`](@ref).
 
@@ -323,6 +331,10 @@ A macro to execute an expression, and return the value of the expression, elapse
 total bytes allocated, garbage collection time, and an object with various memory allocation
 counters.
 
+In some cases the system will look inside the `@timed` expression and compile some of the
+called code before execution of the top-level expression begins. When that happens, some
+compilation time will not be counted. To include this time you can run `@timed @eval ...`.
+
 See also [`@time`](@ref), [`@timev`](@ref), [`@elapsed`](@ref), and
 [`@allocated`](@ref).