Update documentation of tiledb_vfs_touch.

Author: teo-tsirpanis

tiledb/api/c_api/vfs/vfs_api_external.h

@@ -640,7 +640,9 @@ TILEDB_EXPORT capi_return_t tiledb_vfs_fh_is_closed(
     tiledb_ctx_t* ctx, tiledb_vfs_fh_t* fh, int32_t* is_closed) TILEDB_NOEXCEPT;
 
 /**
- * Touches a file, i.e., creates a new empty file.
+ * Touches a file, i.e., creates a new empty file if it does not already exist.
+ *
+ * The access timestamps of the file are not guaranteed to change.
  *
  * **Example:**
  *

tiledb/sm/cpp_api/vfs.h

@@ -519,7 +519,8 @@ class VFS {
         ctx.ptr().get(), vfs_.get(), old_uri.c_str(), new_uri.c_str()));
   }
 
-  /** Touches a file with the input URI, i.e., creates a new empty file. */
+  /** Touches a file with the input URI, i.e., creates a new empty file if it
+   * does not already exist. */
   void touch(const std::string& uri) const {
     auto& ctx = ctx_.get();
     ctx.handle_error(