[docs] Improve the g_get_user_data_dir() docs
[docs] Improve the g_get_user_data_dir() docs

The retrieved directory is XDG_DATA_HOME on UNIX platforms.
Also mention the retrieved directory on Windows: CSIDL_PERSONAL.

bratsche/glib,johne53/MB3Glib,ahmedammar/platform_external_gst_glib,pstglia/platform-external-bluetooth-glib,iConsole/Console-OS_external_bluetooth_glib,krichter722/glib,MathieuDuponchelle/glib,lukasz-skalski/glib,dicer/android_external_glib,bratsche/glib,lukasz-skalski/glib,pstglia/external-bluetooth-glib,dicer/android_external_glib,justinkb/aosp-bluez.glib,tamaskenez/glib,pstglia/platform-external-bluetooth-glib,01org/android-bluez-glib,krichter722/glib,mzabaluev/glib,Distrotech/glib,cosimoc/glib,iConsole/Console-OS_external_bluetooth_glib,djdeath/glib,zsx/glib,MathieuDuponchelle/glib,tchakabam/glib,dicer/android_external_glib,iConsole/Console-OS_external_bluetooth_glib,mzabaluev/glib,endlessm/glib,bluez-android/glib,bluez-android/glib,cention-sany/glib,antono/glib,justinkb/aosp-bluez.glib,ieei/glib-old-android,johne53/MB3Glib,pstglia/external-bluetooth-glib,gale320/glib,zsx/glib,ieei/glib,darren-clark/android_platform_external_bluetooth_glib,ieei/glib,darren-clark/android_platform_external_bluetooth_glib,lukasz-skalski/glib,darren-clark/android_platform_external_bluetooth_glib,pstglia/external-bluetooth-glib,MathieuDuponchelle/glib,tamaskenez/glib,ahmedammar/platform_external_gst_glib,Distrotech/glib,endlessm/glib,justinkb/aosp-bluez.glib,cention-sany/glib,01org/android-bluez-glib,Distrotech/glib,gale320/glib,MathieuDuponchelle/glib,ahmedammar/platform_external_gst_glib,bluez-android/glib,johne53/MB3Glib,gale320/glib,justinkb/aosp-bluez.glib,lukasz-skalski/glib,01org/android-bluez-glib,pstglia/external-bluetooth-glib,01org/android-bluez-glib,ahmedammar/platform_external_gst_glib,johne53/MB3Glib,djdeath/glib,zsx/glib,iConsole/Console-OS_external_bluetooth_glib,mzabaluev/glib,mzabaluev/glib,ieei/glib-old-android,bluez-android/glib,antono/glib,ieei/glib-old-android,gale320/glib,krichter722/glib,gale320/glib,antono/glib,djdeath/glib,darren-clark/android_platform_external_bluetooth_glib,ieei/glib-old-android,ieei/glib,endlessm/glib,cention-sany/glib,tchakabam/glib,djdeath/glib,bratsche/glib,cosimoc/glib,tchakabam/glib,endlessm/glib,pstglia/platform-external-bluetooth-glib,djdeath/glib,tchakabam/glib,iConsole/Console-OS_external_bluetooth_glib,tamaskenez/glib,MathieuDuponchelle/glib,dicer/android_external_glib,cosimoc/glib,endlessm/glib,mzabaluev/glib,tchakabam/glib,bratsche/glib,cosimoc/glib,tamaskenez/glib,01org/android-bluez-glib,Distrotech/glib,zsx/glib,tamaskenez/glib,justinkb/aosp-bluez.glib,krichter722/glib,pstglia/platform-external-bluetooth-glib,johne53/MB3Glib,johne53/MB3Glib,bluez-android/glib,ieei/glib,cention-sany/glib,antono/glib,darren-clark/android_platform_external_bluetooth_glib,cosimoc/glib,ieei/glib,pstglia/external-bluetooth-glib,krichter722/glib,dicer/android_external_glib,Distrotech/glib,lukasz-skalski/glib,cention-sany/glib,pstglia/platform-external-bluetooth-glib