Improve description of @ symbol

As requested by someone on GBDev

src/asm/rgbasm.5

@@ -964,7 +964,7 @@ The following symbols are defined by the assembler:
 .Pp
 .Bl -column -offset indent "EQUS" "__ISO_8601_LOCAL__"
 .It Sy Type Ta Sy Name Ta Sy Contents
-.It Ic EQU Ta Dv @ Ta PC value
+.It Ic EQU Ta Dv @ Ta PC value (essentially, the current memory address)
 .It Ic EQU Ta Dv _PI Ta Fixed point \[*p]
 .It Ic SET Ta Dv _RS Ta _RS Counter
 .It Ic EQU Ta Dv _NARG Ta Number of arguments passed to macro