fixup! fixup! [SandboxIR][Doc] Add a Doc file for Sandbox IR

llvm/docs/SandboxIR.md

@@ -1,6 +1,6 @@
 # Sandbox IR: A transactional layer over LLVM IR
 
-Sandbox IR is an IR layer on top of LLVM IR that allow you to save/restore its state.
+Sandbox IR is an IR layer on top of LLVM IR that allows you to save/restore its state.
 
 # API
 The Sandbox IR API is designed to feel like LLVM, replicating many common API classes and functions to mirror the LLVM API.
@@ -21,7 +21,8 @@ namespace sandboxir {
 
 ## Sandbox IR Value <-> LLVM IR Value Mapping
 Each LLVM IR Value maps to a single Sandbox IR Value.
-The reverse is also true in most cases, except for multi-Instruction Sandbox IR Instructions that may be defined in extensions the Sandbox IR.
+The reverse is also true in most cases, except for Sandbox IR Instructions that map to more than one LLVM IR Instruction.
+Such instructions can be defined in extensions of the base Sandbox IR.
 
 
 - Forward mapping: Sandbox IR Value -> LLVM IR Value
@@ -33,7 +34,7 @@ This mapping is stored in `sandboxir::Context::LLVMValueToValue`.
 For example `sandboxir::User::getOperand(OpIdx)` for a `sandboxir::User *U` works as follows:
 - First we find the LLVM User: `llvm::User * LLVMU = U->Val`.
 - Next we get the LLVM Value operand: `llvm::Value *LLVMOp = LLVMU->getOperand(OpIdx)`
-- Finally we get the Sandbox IR operand that corresponds to `LLVMOp` by querying the map in the SandboxIR context: `retrun Ctx.getValue(LLVMOp)`.
+- Finally we get the Sandbox IR operand that corresponds to `LLVMOp` by querying the map in the Sandbox IR context: `retrun Ctx.getValue(LLVMOp)`.
 
 ## Sandbox IR is Write-Through
 Sandbox IR is designed to rely on LLVM IR for its state.