[NFC][llvm-c] Add full-stop to doc comments

I'd copied and adjusted the doc comments for LLVMGetNNeg and LLVMSetNNeg
in #74517. Nikita pointed out in that review my comments were missing a
full stop, so I'm applying the same fix to these.

Author: asb

llvm/include/llvm-c/Core.h

@@ -3975,13 +3975,13 @@ LLVMBool LLVMGetExact(LLVMValueRef DivOrShrInst);
 void LLVMSetExact(LLVMValueRef DivOrShrInst, LLVMBool IsExact);
 
 /**
- * Gets if the instruction has the non-negative flag set
- * Only valid for zext instructions
+ * Gets if the instruction has the non-negative flag set.
+ * Only valid for zext instructions.
  */
 LLVMBool LLVMGetNNeg(LLVMValueRef NonNegInst);
 /**
- * Sets the non-negative flag for the instruction
- * Only valid for zext instructions
+ * Sets the non-negative flag for the instruction.
+ * Only valid for zext instructions.
  */
 void LLVMSetNNeg(LLVMValueRef NonNegInst, LLVMBool IsNonNeg);