[DOCS] Update directory aliases

[willihay]

Describe the issue

• The @user@ alias now refers to the /user directory by default, it is no longer in the Asset Cache folder
• The @log@ alias now refers to the /user/log directory by default, this is also no longer in the Asset Cache folder
• The @cache@ alias has been renamed to @usercache@ and refers to /user/Cache directory by default.

Impacted page(s)

https://www.o3de.org/docs/user-guide/programming/file-io/#list-of-aliases

[willihay]

Also consider correcting outdated information throughout the File IO topic.