[Wifi] Fix le_wifiClient_GetSignalStrength()
[Wifi] Fix le_wifiClient_GetSignalStrength()

Before the fix:
The API returns the signal strength of the access point got with the
last scan.
After the fix:
- The API returns the current signal strength of the access point.
- Improve the wifi Client Legato documentation without modifying the
interface nor the behavior.

Resolves: LE-10254
Change-Id: Id53e4abe34fe7316b42a58635a008b34eb0b8e94

legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af,legatoproject/legato-af