Clarify screenpt + minor fixes

bios/sys_var_en.u

@@ -161,9 +161,9 @@ This value is also returned by the (!nolink [BIOS]) function Tickcal.
 !label System variable, _v_bas_ad
 !label Logical screen memory, Start of
 !item [_v_bas_ad~~~~LONG 0x44e]
-Pointer to the starting address of the logical screen, which on an ST has to
+Pointer to the start address of the logical screen memory, which on an ST has to
 be aligned at a 256-byte boundary. With an STE and TT this is a 2- or 8-byte
-boundary respectively.
+boundary respectively. This value is returned by Logbase.
 
 !label _vbl_list
 !label System variable, _vbl_list
@@ -497,17 +497,16 @@ are saved.
 !label System variable, scr_dump
 !label Hardcopy routine, Pointer to
 !item [scr_dump~~~~~LONG 0x502]
-Pointer to the hardcopy routine. Is used by 
-the (!nolink [XBIOS]) function Scrdmp.
-Zeiger auf die Hardcopy-Routine. Wird von der XBIOS-Funktion Scrdmp benutzt.
+Pointer to the hardcopy routine. Is used by the (!nolink [XBIOS]) function Scrdmp.
 
 !label screenpt
 !label System variable, screenpt
 !item [screenpt~~~~~LONG 0x45e]
-Pointer to the top of the physical screen. Is transferred at the next
-vertical blank after _v_bas_ad to the relevant hardware 
-registers, bu not cleared subsequently. Therefore one should always work with 
-Setscreen.
+Pointer to the start address of the video memory. It is transferred to the
+relevant hardware registers (returned by Physbase) and to _v_bas_ad
+(returned by Logbase) during every vertical blank (until cleared). As the value
+is not cleared after setting automatically, one should be careful with its usage
+and perhaps use Setscreen instead.
 
 !label seekrate
 !label System variable, seekrate