You can fix this by adopting the PSReadLine module.
See an article on the Hey, Scripting Guy! blog regarding this bash inspired readline implementation for PowerShell.
In particular, the usage of Ctrl+Alt+(Shift)+? to show all keybindings.
PS C:\> $PSVersionTable
Name Value
---- -----
PSVersion 5.0.10240.16384
WSManStackVersion 3.0
SerializationVersion 1.1.0.1
CLRVersion 4.0.30319.42000
BuildVersion 10.0.10240.16384
PSCompatibleVersions {1.0, 2.0, 3.0, 4.0...}
PSRemotingProtocolVersion 2.3
Ctrl+R
PS C:\> Get-Module -ListAvailable
bck-i-search: mo_
Ctrl+Alt+(Shift)+S
Key Function Description
--- -------- -----------
Ctrl+r ReverseSearchHistory Search history backwards interactively
Ctrl+s ForwardSearchHistory Search history forward interactively
In short: either install this module by hand or upgrade to Windows PowerShell v5.