formatting: bulk remove trailing whitespaces

Do not touch vendorized files (e.g. libraries that were imported from external
projects).

No functional changes.

Command:
    find . -name '*.<EXTENSION>' -type f -print0 | xargs -0 sed -i 's/[[:space:]]*$//'

doc/api/changeset_library.md

@@ -70,11 +70,11 @@ This creates an empty apool. An apool saves which attributes were used during th
 ```
 > apool.fromJsonable({"numToAttrib":{"0":["author","a.kVnWeomPADAT2pn9"],"1":["bold","true"],"2":["italic","true"]},"nextNum":3});
 > console.log(apool)
-{ numToAttrib: 
+{ numToAttrib:
    { '0': [ 'author', 'a.kVnWeomPADAT2pn9' ],
      '1': [ 'bold', 'true' ],
      '2': [ 'italic', 'true' ] },
-  attribToNum: 
+  attribToNum:
    { 'author,a.kVnWeomPADAT2pn9': 0,
      'bold,true': 1,
      'italic,true': 2 },

doc/api/embed_parameters.md

@@ -62,7 +62,7 @@ Example: `lang=ar` (translates the interface into Arabic)
 
 ## rtl
  * Boolean
- 
+
 Default: true
 Displays pad text from right to left.
 

doc/api/toolbar.md

@@ -40,7 +40,7 @@ Returns: {SelectButton}
  * {String} value - The value of this option
  * {String} text - the label text used for this option
  * {Object} attributes - any additional html attributes go here (e.g. `data-l10n-id`)
-   
+
 ## registerButton(name, item)
   * {String} name - used to reference the item in the toolbar config in settings.json
   * {Button|SelectButton} item - the button to add