Docs issue: is max file length lines or bytes/chars?

### Description

In both the README and the help file, it's not clear whether `max_file_length` represents lines, or bytes/chars in the file.

### Neovim version

N/A

### Operating system and version

N/A

### Expected behavior

N/A

### Actual behavior

N/A

### Minimal config

```Lua
N/A
```


### Steps to reproduce

N/A


### Gitsigns debug messages

N/A