Improve doc comments for two functions (#13456)

* std.heap: fix function name in doc comment

* std.mem: document return value of `replace`

lib/std/heap.zig

@@ -746,7 +746,7 @@ pub const FixedBufferAllocator = struct {
     }
 
     /// Provides a lock free thread safe `Allocator` interface to the underlying `FixedBufferAllocator`
-    /// *WARNING* using this at the same time as the interface returned by `getAllocator` is not thread safe
+    /// *WARNING* using this at the same time as the interface returned by `allocator` is not thread safe
     pub fn threadSafeAllocator(self: *FixedBufferAllocator) Allocator {
         return Allocator.init(
             self,

lib/std/mem.zig

@@ -2891,6 +2891,7 @@ test "rotate" {
 /// Replace needle with replacement as many times as possible, writing to an output buffer which is assumed to be of
 /// appropriate size. Use replacementSize to calculate an appropriate buffer size.
 /// The needle must not be empty.
+/// Returns the number of replacements made.
 pub fn replace(comptime T: type, input: []const T, needle: []const T, replacement: []const T, output: []T) usize {
     // Empty needle will loop until output buffer overflows.
     assert(needle.len > 0);