Can you identify what in particular the docs were missing?

I agree we don't mention core files moved around, but I don't know why it's relevant; the locations of custom code (mods, settings) hasn't changed.

Thank you Steve for your reply ..

               1) movement of file was one (As i was confused whether the download was half dead @ first then thought might be the upload was corrupted.. Later inspecting the folders one by one i realised that elgg files where present in the folder but still i was confused am i suppose to put only the elgg file or the entire bcz nothing was mentioned in the doc)

2) The setting file must be moved to elgg-config (there where 2 elgg-config files one was at the root the other was inside vendor/elgg/elgg so i was confused where to put the elgg-config inside the root or the vendor) Later i tested it by moving settings & found only the root settings where used by elgg.

3) next must i delete the old elgg 1.x folder that was out for eg _graphics, vendors like those folder i had manually check them by moving inside a new folder & found those files r not necessary.

4) there was 2 mod folders that was also confusing later through discussion i found out that elgg uses the old folder for third party plugins but still i don't know what the other mod folder inside vendor/elgg/elgg is doing.  

5) I don't know if this point is required but i found that deactivating all the plugin was the only way where the egg upgrade worked for me.

It might be simple but when there is 2 folders then mentioning path of the folder is always good i am not a php coder & i work in an office i was not following elgg that close as i was doing before.. I prefered elgg over other CMS bcz elgg is simple to use & the elgg community is alive & helping to solve the problem..

I am here in elgg from 1.8 & i only used the upgrade docs to upgrade but this is the first time i had to go to google to get an understanding of the upgrade. If any of my comments have angered the any elgg core team i am sorry i am just sharing my experience as an end user with a very less knowledge on the php & tech side.

The upgrade was good & the SPEED of ELGG has increased drastically my community is small i have just 2600 active users even i felt the UI had been improved over all the ELGG CORE TEAM has done a marvelous job. 

It sounds like you were over-complicating it for yourself.  Unless you're making changes in core, you shouldn't have to worry or know anything about the layout of the core files.  Simply remove what you have there and replace it with what's in the zip file, and place your custom/third party plugins in /mod

I believe for back-compatibility elgg will even still run with your settings.php in /engine

You shouldn't have to disable all plugins to run upgrade, but a plugin that's not properly forward or back compatible could potentially cause issues I suppose.