docs: clarify command search method for exec and execp.
Fixes
* ext/posix/unistd.c (exec): Clarify there is no command PATH
search.
(execp): Clarify the requested command is found using command
PATH search, like the shell.
@@ -321,7 +321,7 @@ runexec(lua_State *L, int use_shell)
/***
-Execute a program without using the shell.
+Execute a program at exactly *path*..
@function exec
@string path
@tparam table argt arguments (table can include index 0)
@@ -338,7 +338,7 @@ Pexec(lua_State *L)
/***
-Execute a program using the shell.
+Execute a program found using command PATH search, like the shell.
@function execp
@string path
@tparam table argt arguments (table can include index 0)
