docs: getreg() type #34215

```lua
---@type string
local _a = vim.fn.getreg('a', 1)

---@type string[]
local _b = vim.fn.getreg('a', 1, 1)
```

runtime/doc/vimfn.txt

@@ -4089,6 +4089,7 @@ getreg([{regname} [, 1 [, {list}]]])                                  *getreg()*
 
                 Parameters: ~
                   • {regname} (`string?`)
+                  • {expr} (`any?`)
                   • {list} (`nil|false?`)
 
                 Return: ~

runtime/lua/vim/_meta/vimfn.lua

@@ -3684,14 +3684,16 @@ function vim.fn.getqflist(what) end
 --- If {regname} is not specified, |v:register| is used.
 ---
 --- @param regname? string
+--- @param expr? any
 --- @param list? nil|false
 --- @return string
-function vim.fn.getreg(regname, list) end
+function vim.fn.getreg(regname, expr, list) end
 
 --- @param regname string
+--- @param expr any
 --- @param list true|number|string|table
---- @return string|string[]
-function vim.fn.getreg(regname, list) end
+--- @return string[]
+function vim.fn.getreg(regname, expr, list) end
 
 --- Returns detailed information about register {regname} as a
 --- Dictionary with the following entries:

src/nvim/eval.lua

@@ -4569,7 +4569,7 @@ M.funcs = {
 
     ]=],
     name = 'getreg',
-    params = { { 'regname', 'string' }, { 'list', 'nil|false' } },
+    params = { { 'regname', 'string' }, { 'expr', 'any' }, { 'list', 'nil|false' } },
     signature = 'getreg([{regname} [, 1 [, {list}]]])',
     returns = 'string',
   },
@@ -4577,8 +4577,8 @@ M.funcs = {
     args = { 3 },
     base = 1,
     name = 'getreg',
-    params = { { 'regname', 'string' }, { 'list', 'true|number|string|table' } },
-    returns = 'string|string[]',
+    params = { { 'regname', 'string' }, { 'expr', 'any' }, { 'list', 'true|number|string|table' } },
+    returns = 'string[]',
   },
   getreginfo = {
     args = { 0, 1 },