docs: revise batch input requirement for YAML

REQUIREMENTS.md

@@ -9,13 +9,14 @@
 - **raw text mode**: `-t <TEXT>` or `-t "<TEXT>"` (for those including space/tabs); `-t` can be omitted as this is the default mode.
 - **output path**: `-o PATH/TO/SVG`. Automatically append `.svg` suffix if absent. When whole option is absent, use input text but path-incompatible characters substituted with `_`, and filename capped at 32 chars.
 - **batch mode**: `-i <FILE_OF_LINES_TO_CONVERT>` ; output automatically dumps generated svgs per line of input file under `output/`; SVG files named after input lines verbatim except that path-incompatible characters substituted with `_` (underbar), and filename capped at 32 chars. Disregard `-o`.
-- **batch mode with `toml`**: `--toml <LINES>.toml`; the input TOML should be of a flat structure:  
-    ```toml
+- **batch mode with `yaml`**: `--yaml <LINES>.yaml`; the input YAML should be of a flat structure:
+    ```yaml
     item0: text0
     item1: text1
     item2: text2 and there can be spaces
     ```  
     output files go under `output/` and are named `item0.svg`, `item1.svg`, etc.
+    Reject invalid YAML without generating output files.
 
 ## svg generation
-- Generate convex paths, not weighted edges
+- Generate convex paths, not weighted edges