docs: small fixes (#35791)

Close #34938
Close #35030
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Close #35766
Close #35792

Co-authored-by: huylg <45591413+huylg@users.noreply.github.com>
Co-authored-by: Jason Del Ponte <961963+jasdel@users.noreply.github.com>
Co-authored-by: sooriya <74165167+thuvasooriya@users.noreply.github.com>
Co-authored-by: Andrew Braxton <andrewcbraxton@gmail.com>
Co-authored-by: Enric Calabuig <enric.calabuig@gmail.com>
Co-authored-by: Augusto César Dias <augusto.c.dias@gmail.com>
Co-authored-by: David Sierra DiazGranados <davidsierradz@gmail.com>
Co-authored-by: Stepan Nikitin <90522882+vectravox@users.noreply.github.com>
Co-authored-by: Emilien Breton <bricktech2000@gmail.com>

src/nvim/eval.lua

@@ -3854,7 +3854,7 @@ M.funcs = {
     ]=],
     name = 'getcmdtype',
     params = {},
-    returns = "':'|'>'|'/'|'?'|'@'|'-'|'='",
+    returns = "':'|'>'|'/'|'?'|'@'|'-'|'='|''",
     signature = 'getcmdtype()',
   },
   getcmdwintype = {
@@ -3865,7 +3865,7 @@ M.funcs = {
     ]=],
     name = 'getcmdwintype',
     params = {},
-    returns = "':'|'>'|'/'|'?'|'@'|'-'|'='",
+    returns = "':'|'>'|'/'|'?'|'@'|'-'|'='|''",
     signature = 'getcmdwintype()',
   },
   getcompletion = {