Update mmap docs

basis/io/mmap/mmap-docs.factor

@@ -19,6 +19,7 @@ HELP: <mapped-file>
 HELP: with-mapped-file
 { $values { "path" "a pathname string" } { "quot" { $quotation "( mmap -- )" } } }
 { $contract "Opens a file and maps its contents into memory, passing the " { $link mapped-file } " instance to the quotation. The mapped file is disposed of when the quotation returns, or if an error is thrown." }
+{ $notes "This is a low-level word, because " { $link mapped-file } " objects simply expose their base address and length. Most applications should use " { $link "io.mmap.arrays" } " instead." }
 { $errors "Throws an error if a memory mapping could not be established." } ;
 
 HELP: close-mapped-file