Expand on docstring of GC.safepoint (#57128)

Update the docstring to mention the following points:
* Safepoints are very fast, but may still degrade performance in tight
loops
* Safepoints do not trigger the GC, but instead stops a task from
blocking GC that would otherwise run.
* Switch terminology from mentioning 'threads' to 'tasks'

base/gcutils.jl

@@ -243,12 +243,21 @@ end
     GC.safepoint()
 
 Inserts a point in the program where garbage collection may run.
-This can be useful in rare cases in multi-threaded programs where some threads
-are allocating memory (and hence may need to run GC) but other threads are doing
-only simple operations (no allocation, task switches, or I/O).
-Calling this function periodically in non-allocating threads allows garbage
+
+Safepoints are fast and do not themselves trigger garbage collection.
+However, if another thread has requested the GC to run, reaching a safepoint will
+cause the current thread to block and wait for the GC.
+
+This can be useful in rare cases in multi-threaded programs where some tasks
+are allocating memory (and hence may need to run GC) but other tasks are doing
+only simple operations (no allocation, task switches, or I/O), which do not
+yield control to Julia's runtime, and therefore blocks the GC from running.
+Calling this function periodically in the non-allocating tasks allows garbage
 collection to run.
 
+Note that even though safepoints are fast (typically around 2 clock cycles),
+they can still degrade performance if called in a tight loop.
+
 !!! compat "Julia 1.4"
     This function is available as of Julia 1.4.
 """