Document buffer and expr fields in KeySpec API

[Copilot]

Two fields (buffer and expr) were added to the KeySpec API in recent commits but lacked documentation.

Changes

• KeySpec API table: Added buffer and expr field specifications with defaults and descriptions
• Expression Mappings section: Documented expr behavior—clarified that maplayer feeds keys via feedkeys() rather than returning them to Vim's expression evaluator
• Buffer-Local Mappings section: Documented buffer field with examples for direct usage (buffer = true) and dynamic application pattern (make() + autocmd)
• Anchor links: Fixed broken references in API table to newly reorganized sections
• Table formatting: Corrected malformed pipe separators in Chinese README

Example Usage

-- Buffer-local mapping applied to current buffer
require('maplayer').setup({
  {
    key = '<CR>',
    mode = 'n',
    buffer = true,  -- New field
    desc = 'Follow link',
    handler = function()
      vim.cmd('normal! gx')
      return true
    end,
  },
})

-- Expression mapping with feedkeys behavior
require('maplayer').setup({
  {
    key = '<CR>',
    mode = 'i',
    expr = true,  -- New field
    desc = 'Smart enter',
    handler = function()
      return vim.fn.pumvisible() == 1 and '<C-y>' or '<CR>'
    end,
  },
})

Both English and Chinese documentation updated.

---

✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.