core, plugins: fix typos in comments on functions, use imperative

doc/en/weechat_dev.en.adoc

@@ -656,9 +656,9 @@ Example:
 [source,c]
 ----
 /*
- * Checks if a string with boolean value is valid.
+ * Check if a string with boolean value is valid.
  *
- * Returns:
+ * Return:
  *   1: boolean value is valid
  *   0: boolean value is NOT valid
  */
@@ -933,9 +933,9 @@ Example: creation of a new window (from _src/gui/gui-window.c_):
 [source,c]
 ----
 /*
- * Creates a new window.
+ * Create a new window.
  *
- * Returns pointer to new window, NULL if error.
+ * Return pointer to new window, NULL if error.
  */
 
 struct t_gui_window *

doc/fr/weechat_dev.fr.adoc

@@ -660,9 +660,9 @@ Exemple :
 [source,c]
 ----
 /*
- * Checks if a string with boolean value is valid.
+ * Check if a string with boolean value is valid.
  *
- * Returns:
+ * Return:
  *   1: boolean value is valid
  *   0: boolean value is NOT valid
  */
@@ -943,9 +943,9 @@ Exemple : création d'une nouvelle fenêtre (de _src/gui/gui-window.c_) :
 [source,c]
 ----
 /*
- * Creates a new window.
+ * Create a new window.
  *
- * Returns pointer to new window, NULL if error.
+ * Return pointer to new window, NULL if error.
  */
 
 struct t_gui_window *

doc/ja/weechat_dev.ja.adoc

@@ -810,9 +810,9 @@ C 言語のコードを書く際には以下の基本的なルールを *必ず*
 [source,c]
 ----
 /*
- * Checks if a string with boolean value is valid.
+ * Check if a string with boolean value is valid.
  *
- * Returns:
+ * Return:
  *   1: boolean value is valid
  *   0: boolean value is NOT valid
  */
@@ -1085,9 +1085,9 @@ struct t_gui_window *gui_current_window = NULL; /* current window           */
 [source,c]
 ----
 /*
- * Creates a new window.
+ * Create a new window.
  *
- * Returns pointer to new window, NULL if error.
+ * Return pointer to new window, NULL if error.
  */
 
 struct t_gui_window *

doc/sr/weechat_dev.sr.adoc

@@ -653,9 +653,9 @@ WeeChat „језгро” се налази у следећим директо
 [source,c]
 ----
 /*
- * Checks if a string with boolean value is valid.
+ * Check if a string with boolean value is valid.
  *
- * Returns:
+ * Return:
  *   1: boolean value is valid
  *   0: boolean value is NOT valid
  */
@@ -916,9 +916,9 @@ struct t_gui_window *gui_current_window = NULL; /* current window           */
 [source,c]
 ----
 /*
- * Creates a new window.
+ * Create a new window.
  *
- * Returns pointer to new window, NULL if error.
+ * Return pointer to new window, NULL if error.
  */
 
 struct t_gui_window *