strutils: improve doc comments for replace funcs (nim-lang#17427)

This commit fixes mispellings of "occurrence" introduced by:
- 76a3b35 (nim-lang#17337)
- 8e8bea9 (nim-lang#17339)
and adds the same "every occurrence of" in the `replaceWord` func.

Other changes:
- Prefer "replace with" to "replace by".
- Be more consistent with "the" - prefer "of the character" given that
  we wrote "by the character".
- Try to be more consistent with writing the types - add
  "the string `sub`" given that we wrote "the character `sub`".

lib/pure/strutils.nim

@@ -2037,7 +2037,7 @@ func contains*(s: string, chars: set[char]): bool =
 
 func replace*(s, sub: string, by = ""): string {.rtl,
     extern: "nsuReplaceStr".} =
-  ## Replaces every occurence of `sub` in `s` by the string `by`.
+  ## Replaces every occurrence of the string `sub` in `s` with the string `by`.
   ##
   ## See also:
   ## * `find func<#find,string,string,Natural,int>`_
@@ -2079,7 +2079,8 @@ func replace*(s, sub: string, by = ""): string {.rtl,
 
 func replace*(s: string, sub, by: char): string {.rtl,
     extern: "nsuReplaceChar".} =
-  ## Replaces every occurence of character `sub` in `s` by the character `by`.
+  ## Replaces every occurrence of the character `sub` in `s` with the character
+  ## `by`.
   ##
   ## Optimized version of `replace <#replace,string,string,string>`_ for
   ## characters.
@@ -2097,7 +2098,7 @@ func replace*(s: string, sub, by: char): string {.rtl,
 
 func replaceWord*(s, sub: string, by = ""): string {.rtl,
     extern: "nsuReplaceWord".} =
-  ## Replaces `sub` in `s` by the string `by`.
+  ## Replaces every occurrence of the string `sub` in `s` with the string `by`.
   ##
   ## Each occurrence of `sub` has to be surrounded by word boundaries
   ## (comparable to `\b` in regular expressions), otherwise it is not