Clarify that multiple R_RISCV_LO12_I/R_RISCV_LO12_S can share one single R_RISCV_HI20

This is most common on rv32 when loading a 8-byte aligned global
variable, e.g.
```
double NaturallyAlignedScalar = 5.;
double accessNaturallyAlignedScalar() { return NaturallyAlignedScalar; }
//      lui     a1, %hi(NaturallyAlignedScalar)
//      lw      a0, %lo(NaturallyAlignedScalar)(a1)
//      lw      a1, %lo(NaturallyAlignedScalar+4)(a1)

double NaturallyAlignedArray[4] = { 3., 4., 5., 6. };
double accessNaturallyAlignedArray() {
  return NaturallyAlignedArray[0] + NaturallyAlignedArray[3];
}
//      lui     a2, %hi(NaturallyAlignedArray)
//      lw      a0, %lo(NaturallyAlignedArray)(a2)
//      lw      a1, %lo(NaturallyAlignedArray+4)(a2)
//      addi    a3, a2, %lo(NaturallyAlignedArray)
//      lw      a2, 24(a3)
//      lw      a3, 28(a3)
```

The HI20 values for the multiple fragments must be identical and all the
relaxed global-pointer offsets must be in range.

Author: MaskRay

riscv-elf.adoc

@@ -581,6 +581,20 @@ The following assembly and relocations show loading an absolute address:
     addi a0, a0, %lo(symbol) # R_RISCV_LO12_I (symbol)
 ----
 
+A symbol can be loaded in multiple fragments using different addends, where
+multiple instructions associated with `R_RISCV_LO12_I`/`R_RISCV_LO12_S` share a
+single `R_RISCV_HI20`. The HI20 values for the multiple fragments must be
+identical.
+
+[,asm]
+----
+    lui a0, 0       # R_RISCV_HI20 (symbol)
+    lw a1, 0(a0)    # R_RISCV_LO12_I (symbol)
+    lw a2, 0(a0)    # R_RISCV_LO12_I (symbol+4)
+    lw a3, 0(a0)    # R_RISCV_LO12_I (symbol+8)
+    lw a0, 0(a0)    # R_RISCV_LO12_I (symbol+12)
+----
+
 ==== Global Offset Table
 
 For position independent code in dynamically linked objects, each shared
@@ -1533,16 +1547,39 @@ Relaxation result:
   Example::
 +
 --
+Relaxation candidate (`tX` and `tY` can be any combination of two general purpose registers):
+[,asm]
+----
+    lui tX, 0       # R_RISCV_HI20 (symbol), R_RISCV_RELAX
+    lw tY, 0(tX)    # R_RISCV_LO12_I (symbol), R_RISCV_RELAX
+----
+Relaxation result:
+[,asm]
+----
+    lw tY, <gp-offset-for-symbol>(gp)
+----
+
+A symbol can be loaded in multiple fragments using different addends, where
+multiple instructions associated with `R_RISCV_LO12_I`/`R_RISCV_LO12_S` share a
+single `R_RISCV_HI20`. The HI20 values for the multiple fragments must be
+identical and all the relaxed global-pointer offsets must be in range.
+
 Relaxation candidate:
 [,asm]
 ----
-    lui t0, 0       # R_RISCV_HI20 (symbol), R_RISCV_RELAX
-    lw t1, 0(t0)    # R_RISCV_LO12_I (symbol), R_RISCV_RELAX
+    lui tX, 0       # R_RISCV_HI20 (symbol), R_RISCV_RELAX
+    lw tY, 0(tX)    # R_RISCV_LO12_I (symbol), R_RISCV_RELAX
+    lw tZ, 0(tX+4)  # R_RISCV_LO12_I (symbol+4), R_RISCV_RELAX
+    lw tW, 0(tX+8)  # R_RISCV_LO12_I (symbol+8), R_RISCV_RELAX
+    lw tX, 0(tX+12) # R_RISCV_LO12_I (symbol+12), R_RISCV_RELAX
 ----
 Relaxation result:
 [,asm]
 ----
-    lw t1, <gp-offset-for-symbol>(gp)
+    lw tY, <gp-offset-for-symbol>(gp)
+    lw tZ, <gp-offset-for-symbol+4>(gp)
+    lw tW, <gp-offset-for-symbol+8>(gp)
+    lw tX, <gp-offset-for-symbol+12>(gp)
 ----
 --